It basically means "do-over". So when the umpire says "play a let", it means "replay the last rally". Usually used in cases of ambiguity or interference, like if a line judge doesn't see where the shuttle lands, or if a shuttle from another court comes in during play.
The must common let cases, imo, are when the server serves while the receiver is not ready. This is a let and the point is reserved.
Also a good example. If the server serves when the receiver isn't ready, then the umpire calls a let, and the server serves again.
Also if you nth are arguing over wheter or not the shuttle lands in or out in a game without a umpire the players can decide to play a "let" so that the point Is replayed
There's that, and also in this case I'd wager 99% of people know what let means. Not everything can be stickied...
agreed 99.9% of badminton players understand the concept of a "let" and even if they didn't know what the definition of a let is they would still probaly know when it was used or they have probaly still used it without knowing it was called a let