I didn't see this addeessed as it relates to combat sports, or wrestling. I would consider a tap out, or submission to be the opponent quiting. Typically a fighter will tap (rapid, successive tapping on their opponent, or elswhere if they can't tap the opponent), which signifies to the referee/official that they are quitting, and to stop the fight.. There is also such a thing as a "verbal tap", which involves a fighter simply saying "tap, tap, tap". In some combat sports, screaming in pain, is deemed a verbal tap.
A "tap" usually happens when the fighter is subjected to a technique in which is extremely painful, they are unable to breath (such as a choke hold, or sleeper hold) or which will result in a very serious injury, or even CAREER threatening injury. This is typically seen as knee bar, or more commonly, and arm bar, but there are other less common ones. Finally, fighter will tap, if they are about to lose consciousness (submission holds that cut off blood to the brain), although it's not unheard of that a fighter will refuse to tap, understanding that once they lose comsciousness, the referee will stop the fight. On ocasions, you will see a fighter tap due to just getting the crap punched out of them, but they are fighters after all, so I find this to be very rare in professional combat sports. I suppose they are accustomed to getting punched.
There are many YouTube videos showing highlights of the best combat sport submissions in MMA, Jujitsu and Wrestling, as well as other submission sports.