Prime lens is basically a lens with a fixed focal length. Normally it is cheaper to produce due to it’s more basic glass arrangement. Most prime lens have big aperture opening (normally described as fast), such as f/1.8, f/1.4, f/1.2.
One of the cheapest and also the most numerous prime lens is the 50mm. A new 50mm f/1.8 would probably cost less then USD$150 (RM450). I would say that every photographer that have a interchangeable lens camera should have a 50mm.