Submitting by tapping the mat or your opponent's body to signal you give up. In kayfabe, it's admitting you can't take the pain anymore. It's considered a less prestigious loss than a pinfall because you're choosing to quit. Some wrestlers have storylines built around refusing to tap, they pass out in the hold instead. Steve Austin passing out in the Sharpshooter at WrestleMania 13 is the most iconic non-tap in history.
"He tapped! He actually tapped! He said he'd never give up but the pain was too much."