When a professional wrestler finds himself caught in a submission hold, he can give up by slapping the mat with his hand three times in quick succession.

In the good old days (read: before the '90s) tapping out didn't exist.  If you wanted to submit, you had to say "I submit" or "I quit" or "I give up" or something to that effect.  But, as wrestling became increasingly television-centric, it was decided that a more visual way of submitting would be better.  It looks better in a highlight reel, as it's immediately apparent what the wrestler is doing.  Compare that to the old verbal acknowledgement, which could be the wrestler asking what time it is unless you see it in context.  Also, the live crowd does not have the benefit  of watching the match through television cameras five inches from the wrestlers' faces, and so it's easy for them to totally miss a wrestler saying "I submit."

Nowadays, wrestlers will rarely, if ever, submit the old-fashioned way.  Part of it is because of the reasons listed above, another big part is that the wrestlers feel they lose less face when they just tap out as opposed to verbally acknowledging their opponent's superiority.

Just about the only time a wrestler won't submit by tapping out nowadays is in an "I Quit" match, in which the only way to win is to make your opponent say the words "I quit."  The referee will usually carry around a microphone with him so that the entire arena can hear the dialogue when it occurs.