Is En Passant legal?

Could you please clarify for me if the chess move known as 'En Passant' is considered legal within the official rules of the game? I've heard differing opinions on this matter and I'm curious to know the definitive answer. Specifically, how does this move work and under what circumstances is it allowed to be executed? I'm interested in learning more about its legality and how it fits into the overall strategy of chess gameplay.