What does Luke 13:4 mean?

A large crowd learns from Jesus that they need to repent of their sins and reconcile with God because they never know when they might die. News is spreading that Pilate killed some Jews from Galilee. Apparently, they had gone to Jerusalem to offer sacrifices, possibly for Passover. Jesus points out that this tragedy does nothing to prove the dead did something wrong. Their deaths were the result of terrible, unjust violence, and anyone in the crowd could meet a similar fate (Luke 13:1–3).

Now, He mentions another example. A tower at a reservoir in Jerusalem had fallen, killing eighteen people. Like Pilate's victims, those killed by the tower had done nothing to deserve this. "Offender" refers to someone who is in debt. This was not God's punishment for sin. God did not corral the most sinful people in Jerusalem under the tower and then use a finger to knock it over. While the Lord is in control of all things, not everything that happens to a person is a response to their own personal sin: the tower collapse was a spontaneous tragedy.

Jesus' point is that people die suddenly every day. You can't escape death. What you can do is make sure you are right with God so that after physical death He will bring you home to Him. One day there will be no more tears, violence, or senseless tragedies (Revelation 21:4).

Neither the Bible nor any extra-biblical writings give more details about Pilate's violence against the Galileans or the falling tower of Siloam. That doesn't mean these events didn't happen, just that in the grander context of the Roman Empire, they were minor events.