What does Genesis 11:5 mean?

The united peoples of earth make a plan to exalt themselves, by building a tower in their city that would reach to the heavens. Instead of asking God for help, they set out to make themselves powerful enough to get what they wanted. Many scholars see this as a deliberate attempt to guard against another flood, by making a structure specifically intended to protect mankind from God's judgment.

This verse is probably intended to be taken in a dry, almost sarcastic tone. The text reveals that God chooses to "come down" to even see their tower. Of course, God did not need to travel any distance in any direction to see or know what was happening on the earth. Instead, the language is meant to emphasize what a silly idea it was that people could build a tower that would reach to the heavens, or prevent God from enacting His will.

In a more direct sense, this verse shows God "playing along," much as He did when Adam hid from Him in the garden of Eden and God called out (Genesis 3:8–10).