In many programming languages, a "primitive" is a basic data type to hold simple information, i.e. an integer or a character. When I first started programming I gave myself the constraint to only use primitive shapes, allowing any complexity to come from the logic in the underlying code. Whenever I working on a new technique or idea, I will often create a primitive sketch as a tool for exploration.

Primitives is an ongoing series of sketches created in Processing. See all the Primitives here.