What is a "Miss" ?

Never seen that word before in my analysis, I think it's considered a blunder, but what is the difference between a "Miss" and a regular "Blunder ?

I just noticed this yesterday... It seems to have replaced "missed win", but is listed before blunders instead of after them.  But the way that the eval bar jumped for "miss" seems like a blunder.

It's a new classification that replaced Missed Win and also includes things other than just that. There was also a change to the Blunder classification.

1. Improved Blunder Classification - It used to be that if the engine evaluation went down a lot, then you’d get a blunder. However, this isn’t how humans tend to evaluate. Engine evals can drop due to weird positional considerations that are hard to understand and require long-term precise play from your opponent to punish. In contrast, blunders are typically considered very immediate and concrete. The new blunder definition requires your move to lose material or allow a forced checkmate, in addition to the large drop in engine eval.
2. New Miss Classification - Sometimes a move is bad not because something went horribly wrong with your own position, but because you failed to punish your opponent’s bad move. For example: you’re in an even game, your opponent gives you the opportunity to fork and win a piece, but you just make a normal continuing move as if nothing happened. These cases will be covered by a new classification called Miss, which will be denoted with a Ø glyph.

i thought it was a missed checkmate chess please tell me if im right

Not necessarily, although this is one classification.