What does collectible systems mean?

Field: psychology

If ideas or concepts can be collected together they are collectible.

Your quote is from the Wikipedia article about Professor Baron Cohen's empathizing-systematizing theory of autistic brain types. (See, I fixed the context for you)

Individuals with autistic spectrum disorder may have superior skills in systematizing - that is, analyzing or constructing systems to understand the world. They do this by noting regularities, structures and rules within systems. This leads such individuals to focus on fully understanding individual systems. A system is anything that follows rules. It might be

a mechanical system (e.g., a machine or a spinning wheel)

an abstract system (e.g., number patterns, the syntax of a language, timetables)

a natural system (e.g., water flow, or the weather)

a collectible system (e.g., classifying objects such as DVDs by author or toy cars by shape, colour, size).

Simon Baron Cohen - Borat's cousin! (And Ali G's)