Shield-stun is the amount of time a shielding fighter is forced to stay in a blocking animation. This forced state cannot be avoided, even if the defender releases the Shield button.
Attacks inflict varying amounts of shield-stun. Generally, stronger attacks inflict more, although this isn’t a universal rule. Certain rapid-fire attacks (like Sheik’s neutral attack 3 loop) can keep foes locked in shield-stun until the shield pushback pushes them out of range, since the strikes occur so quickly.
Chainable attack sequences, like Cloud’s Cross Slash series, have very low shield-stun, so it’s possible for the defender to drop their shield in between hits. Either player can use this to their advantage in these types of situations, depending on timing and reads. For instance, if a Cloud player guesses that the opponent will drop their shield immediately and try to counterattack, chaining into the second hit of Cross Slash will pre-empt the enemy. On the other hand, an opponent who correctly reads when Cloud will cease his attack sequence can attack with up tilt out-of-shield, launching Cloud during his end lag if correct.
A fighter’s shield-stun can be completely nullified with perfect shield or by shield-grabbing. Shield-grabbing can be a risk, though, especially with fighters who have limited grab range. Whiffed grabs are usually punishable.