A component of speed, reaction time refers to the time it takes a character to register that something is happening and respond (thus, the time to react). A common way of judging reaction time is through bullet-timing feats. A character with sufficiently low reaction time (high reaction speed) may be able to fend off a speedblitz, even if said character cannot travel that fast.
While movement/travel speed and reaction speed are different, it should be kept in mind that they are almost always related. Characters with superhuman reaction times sometimes don't have superhuman movement speed, but it's very rare for a character with superhuman movement speed to not have superhuman reaction times.
For example, let's say you could run at the speed of sound, but could only react as fast as you normally can. Unless you're in the middle of a desert or huge plain with nothing around, then as soon as you start running you'll likely smash into something and hurt/kill yourself. Certain trolls tend to assume that movement speed and reaction speed are completely independent, and claim that reaction speed can only be proven by fights (and some go even further to say that fights that occur while the characters are traveling long distances don't count). Of course the reality is that any character with superhuman travel speed is likely going to have superhuman reaction speed as a matter of necessity. Not that there aren't exceptions, but they're rare.
Reaction time is one of the most commonly calculated feats in the OBD, since the equations are rather simple (reaction time = distance the character was from the thing they were reacting to when they first reacted / speed of the thing the character is reacting to. For example, if a character reacted to a projectile moving at 100 meters/second when it was 1 meter away from them, their reaction time would be 1/100, or 0.01 seconds).
Of course it's almost always more complicated than that, but this was just a basic example to show you how the general idea is supposed to work.
- Combat Speed Fallacy