En passant?


https://www.chess.com/terms/en-passant
You use it when it's the best move.

"En passant" is a French phrase meaning "in passing." How it works: let's say you have a pawn on c5. You want to capture your opponent's b pawn when they play b3, which you're expecting. But they play b4. Is the chance to get the pawn lost? No, because you can play cxb3 e. p. You put your pawn on the square b3 and you remove the pawn on B4. Why is it used? The same reasons as any other pawn capture: to get your pawn closer to being promoted; open up a rank, file, diagonal, break up their King's pawn wall; discover an attack on an enemy unit when your pawn moves out of the way *and* the pawn itself is attacking something. You only have one chance to do it; if you don't do it immediately you forfeit the option. There's a spot on this site which explains the rules of moving units. I *may* have left something unsaid or made an error in my post. Hope this helps!

It's extremely bad manners to post dumb memes when a beginner asks for help.

For your last question, I don't recall seeing it as a possibility in every game I play, the need to play it may not always arise and I can't say I've always noticed a difference in my games. It might tend to surprise and/or unsettle my opponent because they overlooked the possible move, especially if it creates a sudden crisis for them. Otherwise 🤷