There are a couple of mechanics that are common among all smash games. We'll start with the following:
Referred to in the game code as fallspecial, Helpless occurs when you recover (up special) or use certain side or down special moves in the air. The character will stay in Helpless state, only able to move left or right, and will only leave the Helpless state upon landing on the game stage or platforms, grabbing a ledge, or dying to a blastzone.
In Melee, air dodging will also leave a player in a Helpless state, but this was removed in Brawl and all subsequent Smash games.
Each character has different timing for when Helpless activates, based on the special move used, so I'll stay generic with it for now. Later Smash games have changed the movesets of certain characters to remove Helpless states.
You can read more about the Helpless state here.
End Lag refers to the number of frames that any given move has to go through before another move can be inputted. Every move is affected by End Lag to some degree, some more than others. For example, slower attacks from characters like Ganondorf and Bowser tend to have a lot of end lag, especially in older games where those characters tend to be lower-tier and weaker. Part of the reason they are low tier is the End Lag on all their moves, preventing them from attacking faster and creating combos.
Characters like Meta Knight, considered far and wide to be such a good character in Brawl that he has his own tier in tier lists, have almost no End Lag, allowing them to quickly perform moves one after another, even specials.
You can read more about End Lag here.