"Tap Out" is a wrestling term referring to the preferred method of submission, or yielding to ones opponent. To submit in a wrestling match, one taps their hand three times on the mat. As the other respondent has said, it's also the name of a tv program centered on mixed marital arts competitions.
Its one of the biggest brands that endorse mixed martial arts in leagues such as UFC and PRIDE. Many fighters choose to wear this, but at the same time a lot of punk kids think it will make them look tough if they wear this.