The idiom `马后炮` is deeply rooted in the strategic game of Chinese Chess (象棋, xiàngqí), a game with a cultural significance in China comparable to chess in the West. The vivid imagery of a checkmate makes the meaning instantly clear to anyone familiar with the game. Culturally, giving a `马后炮` is considered poor form. It can be seen as an attempt to appear wise without having taken any of the risks involved in the actual decision-making process. It can cause the recipient to “lose face” (面子, miànzi) by highlighting their failure in a non-constructive way. Chinese culture often values providing support and helpful suggestions before an action is taken, rather than criticism after.
`马后炮` is a very common term used in informal, everyday conversation. It's almost always used with a negative or sarcastic tone to complain about someone's unhelpful commentary.