Trying to understand a pawn move

I have recently started playing chess again (been quite a number of years), and I was playing the computer this morning and there was a pawn move I had never seen before.  I tried to look it up online but I could not find it described in any of the places I looked.   Ignore the opening game, still learning how to use this tool.

Black advances 2 spaces to h5, White castles kingside O-O, then the computer plays gxh5 (I can't get the sample game board to play that move, but I tried it myself in human v human analysis mode and as long as the sequence is black advances any pawn 2 spaces, white castles, then black can move a pawn into the skipped square left behind by the first pawn.

Can someone please help me out here, I am going crazy to understand this move.

It sounds like you are talking about the En Passant rule, although that isn't evident form the diagram you give:

https://en.wikipedia.org/wiki/En_passant

Does this clear it up for you?