Yagni means You Aren't Going to Need It
The translation You Ain't Gonna Need It is also used frequently and it is a principle that comes from the software corner. It's about that you only have to make things that are used, and if you expect or know that something is rarely or not used you should not make it either. It's also about the principle that something simple that works is often good enough, it doesn't have to be very beautiful and shiny: if it works.