Invincibility

An Invincibility frame(s) is the period of time where a character is completely invulerable to everything. It's not to be confused with knockback resistance, which only reduces or negates the effect of knockback from an attack, while the character will still take damage from said attack. When a character is in invincibility frames, he/she/it cannot be grabbed, take damage, or be knocked back.

A player can gain invincibility frames in a number of ways. By sidestepping (pressing down while shielding), a character will gain several invincibility frames, for about one second, while remaining in one place.

By rolling (pressing directional keys while shielding), it is the same as sidestepping, except the character will briefly move left or right depending on what key was pressed.

If a player presses the shield button and does not use any other moves while airborn, the character in his/her control will perform an air dodge, granting them temporary invincibility in the air.

Characters can also gain invincibility frames in other ways. An example can be during the initial stages of a character entering their Final Form or using some other form of a Final Smash, which grants them invincibility frames as they transform or exceute their attack to prevent interruption.

Some attacks will also grant the user of them invincibility frames when they are exceuting said attack. For example, when Kyūbi Naruto uses his Side Special Move Bijūdama, he gains invincibility frames during the time he is in his Four-Tailed Form. When Super Saiyan Goku uses his Down Special, Genki Dama, even though he had gone back to his normal form, he still gains Invincibility Frames during the time he is charging the ki ball. Fire Mario's Mario Finale is yet another example.

When a character is KO'ed and gets off of their revival platform, they will have about two seconds (120 frames) of invicibility as well, although it's likely the actual period of Invincibility is a bit longer than that.