What does John 10:18 mean?

The context of these remarks is important: Jesus is still debating with religious critics who are angry over His recent miracle (John 9). There, Jesus gave sight to a man born blind, which sparked debates that did not end well for the local religious leaders.

Now, Jesus continues to explain His role as "the good shepherd" (John 10:11, 14). That includes His willingness to die for the sake of His sheep. That sacrificial love is a reason Jesus has special favor with God the Father (John 10:17; Philippians 2:8–9). It's possible those listening assumed Jesus' prior statement was just an assumption. In other words, that Jesus was "willing" to die, not that He "would die." Talk of Christ's death is something Jesus' closest followers often struggled to accept (Mark 8:31–33).

As He continues, Jesus makes it clear that His role as "the good shepherd" (John 10:10–14) and "the door" (John 10:7–9) not only includes an actual death, but it also includes resurrection. That death is entirely voluntary; it is not something into which Jesus is being coerced (Matthew 26:53). And it will result in a resurrection, based on divine power and authority (John 2:19–21). In this relatively brief statement, Jesus claims to have power over life and death—even His own—as granted to Him by God. He predicts His own death and resurrection.

The grand nature of those ideas may be a reason that this specific audience doesn't seem to react with accusations of blasphemy, as they do in other passages (John 5:18). What Jesus says is so outlandish that it suggests two other possibilities. The audience seems torn between Jesus being possessed—the ancients' reference to insanity—and being a miraculously-verified messenger (John 10:19–21).