In Bible times, shepherds kept their sheep in a pen at night. In the morning the shepherd called them, and they followed his voice. Jesus used this picture. He is the door of the sheep, the only way in. Those who enter through Him are saved and find pasture. Thieves come to steal and destroy, but Jesus came so His sheep may have life, and have it abundantly.
Then Jesus said, I am the good shepherd. A hired hand cares about pay, not the sheep, and runs when the wolf comes. But Jesus knows His sheep, and they know Him. He would lay down His life for them by His own choice, and He would take it up again. He also has other sheep, not of this fold, and He will bring them in too. That includes Gentiles, people from every nation.
Some people asked Jesus to tell them plainly if He was the Christ. He said His works showed it, but they did not believe because they were not His sheep. His sheep hear His voice and follow Him. He gives them eternal life, and no one can snatch them out of His hand or the Father's hand. When He said He and the Father are one, they tried to stone Him. He went away across the Jordan, and many believed there.