"Showing off" something means showing people something good that you have or that you did.

You can "show off" things like:

• your nice house
• your guitar-playing skill
• your very handsome boyfriend

You use the phrase like this:

I want to show off the necklace I got.

You can also use "show off" by itself:

He's always showing off.

In this case, "showing off" means acting in a way to make people impressed with you.