It implements defining words ie. Additional compiler words.

It does this by doing something now, and later (you could read create does> as now later>)

So for

: CONSTANT CREATE , >DOES @ ;

This makes the defining word CONSTANT, which when run (now) compiles the next word

So CONSTANT myvar will compile myvar. Myvar, when run (later) will get it's value and push it to the datastack.

It only briefly goes into what it does, this article goes into how it's done for a particular implementation.