Speed (mobility, etc.) is one of the properties a character has. It indicates their ability such as the velocity of their motion, the speed and range of frontsteps and backsteps, the existence and efficacy of teleport moves and things like them, and so forth.
Grapplers, being characters centered around throws, have a tendency toward high offensive outputs and defensive ability as well as low speed, jump ability, and throughput (though recently there have been those with just opposite tradeoff).