Knockback resistance, known by some as super armor, is a phenomenon that manifests itself during certain moves in all three Super Smash Bros. games. It is defined as a character not flying as far when hit during a certain time, or in some cases not even flinching. Note that knockback resistance, as the name implies, only affects knockback. The attack's full damage will still be taken.
As an example of how knockback resisting works, take Yoshi's double jump. During his upward movement, Yoshi gains (approximately) 120% knockback resistance. This does not mean he flies 120% less distance when hit. It means that when hit, his flight distance is calculated from his current percentage minus 120%. So if Yoshi is hit with 150% damage during his double jump, he wil fly as far as he would if he had 30% damage. If the knockback resistance calculation's result is negative (say -50%) then Yoshi would not even flinch.
In SSBM, Yoshi retained his resistance, while Metal and Giant characters had their resistance decreased a whole lot. Also, Giga Bowser has about 50% knockback resistance when fought in Adventure Mode.
In SSBB, many characters inherit knockback resistance, often in the form of moves that give a temporarily infinite amount: