Jesus started the analogy on shepherds and sheep, but the audience did not understand the analogy. As a result, Jesus explained what the analogy is about.
First, Jesus declares himself as "door of the sheep" (10:7). The door is meant to be the legitimate way for entry. He contrasts himself to others who were thieves and robbers. Those people were people before Jesus who claimed to be Messiah (Savior) but did not have the credentials. The sheep does not hear them because those people did not have the proper credentials.
However, while the sheep reject the thieves and robbers, the sheep can go through the door and find pasture (10:9). The door provides an access point because it involves a route of legitimacy. In a similar way, Jesus has the authority that the "sheep" (AKA people who trust God) can recognize.