What does Acts 5:6 mean?

We don't know why, exactly, God killed Ananias. Possibly the main intent was to set an example of holiness in the church. But the incident does show how God feels about dishonesty in the church. There is no need to make ourselves appear spiritually better than we are. To lie in a setting where the people are specifically told not to judge is the height of foolishness (Romans 14:4).

One of the primary laws of interpreting Scripture is to do so in the context of other passages. Reading nothing but this short passage, God might appear harsh and capricious. He seems to judge Ananias without warning. This knee-jerk reaction would not be consistent with the rest of Scripture. God gave Adam and Eve clear instructions that they deliberately disobeyed (Genesis 2:17). The Bible doesn't give specifics, but apparently Noah warned the people of coming judgment (2 Peter 2:5). God even sent Jonah to the evil Ninevites fifty years before finally telling Nahum to announce He was done with them. For hundreds of years God sent prophets to warn the kingdoms of Israel and Judah before He sent them into exile.

Second Peter 3:9 says God is patient with the rebellious, giving them time to repent and seek forgiveness. Although the text doesn't say how God interacted with Ananias and Sapphira before this event, it is reasonable to assume He did not condemn them for a single spontaneous act. If Ananias and Sapphira were with the others, learning from the apostles (Acts 2:42), they should have known better. But Judas proved that no amount of teaching can change a hardened heart.