What does John 10:5 mean?

Sheep in Jesus' era were acutely attuned to the voice of their own shepherd. This allowed multiple flocks to be housed in a single sheep pen overnight. Individual shepherds could retrieve their flocks simply by calling. Only those sheep who knew that specific voice would come; the others would not respond to a voice they did not recognize.

By analogy, Jesus is explaining that the reason many of His critics cannot—actually, will not—respond to Him is because they are part of another flock. Those who "belong" to Jesus recognize His voice and follow Him, while recoiling from the voice of a stranger. Those who don't listen to Jesus' voice are effectively "owned" by someone else. As Jesus pointed out in prior conversations, such people belong to Satan (John 8:42–47).

This is the first of three separate analogies Jesus will use that involve shepherding. As the following verse shows, the men to whom He speaks—the same religious leaders who criticized His healing of a man born blind (John 9)—will predictably fail to get the point.