En passant is a strange rule that allows a Pawn to capture an opposing Pawn taking advantage of the 'two square rule'. In the diagram on the left, White's Pawn just advanced two squares, allowing Black's Pawn to capture it en passant for one move only.
Black took White's Pawn en passant, playing dxc3 e.p. En passant is a French phrase, so nobody really knows how to pronounce it, but the closest we can get to it is 'auh pahs-sah'. The literal translation into English is 'in passing'.