To add to Neil's answer, the expression "to show off" relates to being boastful and not humble.
For example, if someone is the best player on their sports team they can be humble by saying, "Even though I play this sport well, they are definitely better players out there." If that same person wanted to show off they could instead say, "I'm the best player that ever existed! If you don't agree then you don't know how to appreciate true talent."
A classic "show off" saying is "anything you can do, I can do better."