The Parable of the Good Shepherd

Jesus said: "Truly, truly, I say to you, he who does not enter the sheepfold by the door but climbs in by another way, that man is a thief and a robber. But he who enters by the door is the shepherd of the sheep. To him the gatekeeper opens. The sheep hear his voice, and he calls his own sheep by name and leads them out. When he has brought out all his own, he goes before them, and the sheep follow him, for they know his voice. A stranger they will not follow, but they
will flee from him, for they do not know the voice of strangers." (John 10:1–5). 

Jesus is using a metaphorical language. In this speech, Jesus is the shepherd, see Psalm 23: The Lord is my Shepherd. He is also the door for the sheep.

When a shepherd takes care of his sheep in Jesus' day, at the end of the day he would lead his sheep to a sheepfold, where the sheep spend the night. Such a sheepfold would often be a place where several different flocks would come. Then there would be a watchman or gatekeeper who would basically stay there all night long, to protect the sheep and prevent thieves or animals from coming in to harm them. 

When the shepherd wakes up in the morning, he will go back to the sheepfold where the gatekeeper will recognize him as one of the shepherds. The shepherd will go in where the door (opening) is and get his sheep. He does it by calling his sheep. The sheep listen to his voice. (They didn't mark their sheep to be able to distinguish which flock they belonged to.) A person who goes any other way is a thief.
See this explained at time 7:45–9:24 in John 10:1-21 – Opposed by Wolves.

What Jesus is saying is that there are leaders who try to lead the flock or take control of the flock to walk in an inappropriate way. Since the gatekeeper doesn't know him, he has to choose another way into the sheepfold. There is a real way to reach the sheep. And it's by being a shepherd and entering where the door (opening) is via the gatekeeper. A good shepherd leads the sheep, he doesn't drive them out. The sheep follow the shepherd because they know his voice. They know they are safe with him and follow him. The best way to get to know the voice of Jesus is to study his Word, the Bible, not become prideful, and to continue studying
the Bible. A bad leader tries to control the flock, see also The Book of Timothy.

Jesus explained this parable in the following way: "Truly, truly, I say to you, I am the door of the sheep. All who came before me are thieves and robbers, but the sheep did not listen to them. I am the door. If anyone enters by me, he will be saved and will go in and out and find pasture. The thief comes only to steal and kill and destroy. I came that they may have life and have it abundantly. I am the good shepherd. The good shepherd lays down his life for the sheep. He who is a hired hand and not a shepherd, who does not own the sheep, sees the wolf coming and leaves the sheep and flees, and the wolf snatches them and scatters them. He flees because he is a hired hand and cares nothing for the sheep. I am the good shepherd. I know my own and my own know me, just as the Father knows me and I know the Father; and I lay down my life for the sheep.
And I have other sheep that are not of this fold. I must bring them also, and they will listen to my voice. So there will be one flock, one shepherd. For this reason the Father loves me, because I lay down my life that I may take it up again.
No one takes it from me, but I lay it down of my own accord. I have authority to lay it down, and I have authority to take it up again.
This charge I have received from my Father." (John 10:7–18, Gal 3:28).

Jesus is the door for the sheep, see the meaning of this this explained at
time 21:51–26:25 in John 10 (Part 1) :1–21 • “I am the Good Shepherd”. 
When a shepherd was out in the field and far from the sheepfold, he could leave the sheep in, for example, a cave during the night. He himself would then lay down on the ground in front of the cave and be the door to the cave where the sheep were. 

When Jesus said that the good shepherd lays down his life for the sheep, he is referring to his coming death on the cross, see The Crucifixion of Jesus. This is a picture on good leadership that Jesus exemplifies for us. A good leader must be ready to lay down his life for his flock and not be too interested in preserving his own life. A godly shepherd thinks of the people first, see The Book of Titus. It comes from brokenness, see the last part of The Apostle Peter. See this further explained at time 19:56–22:34 in John 10:1-21 – Opposed by Wolves. 

Jesus also speaks of the church, the body of Christ, that consists of both born again Jews and born again Gentiles (John 10:16). We all belong to the same flock, with one shepherd Jesus Christ. Under Jesus we also have other shepherds
in the body of Christ (Eph 4:11), see The Book of Ephesians.

Jesus gave his life willingly (John 10:11–13), see time 29:26–33:51 in 
John 10 (Part 1) :1–21 • “I am the Good Shepherd”. Jesus had the authority to
put it down, but also to pick it up again (John 10:17–18). All three persons in
The Trinity attended in The Resurrection of Jesus.

The Shepherd's voice lives within us through the Holy Spirit. He is there to confirm and cooperate with God's Word that we hear or read. We then need to be attentive, listen, and follow his guidance. By applying that and by studying the Bible, 
we can verify whether what we hear or read is correct. If we don't, there is a risk that we will forget his words and begin to follow false teachers and leaders.

The purpose of the thief is to come and take, not to come and give (John 10:10a, Acts 20:29, Mat 7:15). He may want to get from the flock; love, worship, power, money, recognition, see the end part of The Book of John. The hired hand is the other threat. It can be a person who only looks after an income from the ministry. When danger approaches, he leaves the flock (John 10:12). It's not wrong for a teacher and leader in the body of Christ to receive a decent salary (1 Tim 5:17–18,
1 Cor 9:9, 13–14, Luk 10:7, Deu (5 Mos) 25:4). But it's more than that. It's a person who first and foremost cares about himself (John 10:13, Ezekiel 34:1–6).

The perfect example of leadership is Jesus Christ (Psalm 23:1–6), see
Who is the Greatest in the Kingdom of Heaven? 

For information on other parables, see Jesus' Parables.