(I will attempt to do this topic justice, but honestly, there is a much better treatment on this subject at every-layout.dev)
Most subject areas have a concept of primitives or fundamentals. Ask any of the masters of music, art, language, food, or athletics and you will find that the amazing things they do are the result of mastering the primitive skills of their area and learning how to combine them in a way to make powerful things.
In programming languages the term primative has a special meaning. Strings, Booleans, and Integers are all examples of primitives in many common programming languages. These primitives are the lowest parts of a programing language. By themselves do not do much; however, through composition, primitives can be used to create amazing things.
In that same vein, Layout primitives are the lowest-level building blocks of a layout language. In and of themselves, they can't do much. It is through the composition of these layout primitives, full application layouts can be made.