Freeze frames, (also known as hitlag, officially known as impact stall), are a phenomenon that appear in all four Super Smash Bros. games whereby a character will freeze for a period of time after connecting with an attack. If successful with some attacks, the attacker is frozen in time for a number of frames while the target suffers from hitstun. This causes the attack to get "stuck" out longer than it would if it had not hit anything, while the target has a short time to react before the knockback occurs. This is most easily noticed in multi-hit attacks such as Yoshi's d-air; the attack takes less time to finish if it does not hit anything.
The number of freeze frames, that is, the length of the freeze, depends on the attack - weak, normal attacks such as Mario's neutral punch have minimal freeze frames, but the freeze frames of strong, electric, and sweetspotted attacks can last as long as half a second. Freeze frames are also exaggerated if two attacks clash, or the attack is Perfect Shielded. In the latter case, the attacker suffers from hitstun while the defender receives none.
Freeze frames only affect the object that deals the damage; all other game elements are uninterrupted. For example, Captain Falcon will be frozen for half a second upon a sweetspotted Knee Smash, while Samus' movement is not interrupted by a Charge Shot hitting something. Freeze frames affect the attacker as long as the attack connects, even if it deals no damage. Thus, if a character were to attack an opponent with blocking invincibility, that character would still suffer freeze frames.