What does John 16:12 mean?

In this passage, Jesus has been explaining the coming and work of the Holy Spirit (John 16:5–7). That includes the ways in which the Spirit will "convict" the unbelieving world (John 16:8–11). A main reason Christ has made these comments is to prepare His disciples for the hard times to come (John 16:1–4). In the next few hours, they will see Him arrested and crucified (John 18:1–3; John 19:18). After Jesus is resurrected, they will have the help of the Holy Spirit (John 20:22), but they will still be subject to persecution. Knowing these hard times were predicted gives Christians reassurance that God is still in control.

A recurring theme in the Bible is that God understands our weaknesses. Jesus, in particular, can fully sympathize with human temptation since He experienced a human life (Hebrews 4:15–16). The recent discussion of Jesus leaving, and of hard times, almost certainly made the disciples nervous and afraid (John 16:6). Rather than overwhelm them with even more, Jesus knows when to stop. This, again, is one of the reasons God will send the Holy Spirit to encourage, teach, and remind these men of those things they can't yet fully understand (John 16:13).