Lag refers to any period of time when a fighter is occupied with a current action without an actual active threat of some kind extended. Lag can refer to a move being sluggish in general, or it can specifically call out slow starting or ending lag, highlighting where the windup or follow-through of a move is extra vulnerable. Lots of starting lag on an action means you’re much more likely to get interrupted before it ever comes out. Lots of end lag means you’re much more likely to get punished after the active period on whiff or block.
Landing on solid ground after being airborne has its own kind of lag, called (surprise) landing lag. There’s always a tiny bit of landing lag; the smallest amount is for jumping and doing nothing (an empty jump or hop). A neutral air dodge just before landing has slightly more landing lag than that; an angled air dodge just before landing has quite a bit more. Air moves vary wildly, and how much landing lag an air move adds near the ground (and depending on when it was used before landing) often helps determine whether it’s a good short hopping move.