GM move or a bug?

Game goes like this, yes I am poor skilled player (practise practise).
1. e4 / e5
2. c4 / c6
3. d3 / d6
4. h4 / h5
5. knight f3 / knight f6
6. bishop g5 / bishop e7
7. knight bd2 / d5
8. knight xe5 / dxe4
9. c5 / exd3
10. knight dc4 / b5
11. cxb6 ? > but there is no soldier on that spot... it is in b5..

game is against computer, lvl5

This is called "en passant" and is part of the chess rules.

Whenever a pawn moves two squares, you can basically capture it with a pawn of yours as if it had moved only one square, but only with a pawn and only in the move directly after the pawn's "double move".

Learned something new again, cool. Thank you!