The Street Fighter terminology, acronyms and lexicon guide here on EventHubs.com has been fleshed out a bit more.High or Low Priority: Priority is best explained with an example. In Street Fighter Alpha 2 Rose's Crouching Medium Punch is considered to be a high priority move. If she executes this attack from close range around the same time Guy attempts to do a Crouching Hard Kick, Rose will hit Guy out of his attack.
There are many reasons why moves will beat out one another. The start up time it takes for a move to come out, how many frames it's active for (in a hittable state), where the players hit boxes are at and if they have full or partial invincibility to attacks are the core reasons.
Generally moves with 'high priority' have a fast start up time, a solid window they're active for and they make the character's hittable box smaller so it's difficult to counter — although there are other properties that make a move high priority as well.
So priority is best thought of not as entity on to itself, but the actual properties of a move.
