Priority

Priority is a property of every attack in the Super Smash Bros. games. In general, priority refers to an attack's ability to trump another attack if both are used at the same time. Priority is usually related to the damage the attack deals.

For example, if two Marios punch each other at the same time, the attacks will "cling" (that is, both attacks will be stopped) and neither will take damage. This is because the attacks have the same priority; this can also happen if the priority of the two attacks are similar but not completely equal. But if Mario's weak punch were to collide with Yoshi's forward smash, then Yoshi's attack (with its higher priority) will go right through Mario's attack, damaging him without Yoshi taking damage.

If two characters attack each other in the air at the same time, then they can hit each other regardless of priority (unless one of the two attacks comes out faster). Priority only affects grounded characters.

Generally, more damaging attacks have higher priority: a forward smash will almost always trump a jab, for example. The main exception to this is grabbing: a grab always has more priority than an attack.