What is BUK?

It is difficult to provide a precise explanation of 'BUK' without more context. 'BUK' is commonly associated with the Russian-made Buk missile system (also known as the 'Beech' system). This is a family of self-propelled, medium-range surface-to-air missile systems. It's designed to engage aircraft, cruise missiles, and precision-guided munitions. Without more information, I can only assume the search term refers to this.