No, it more or less means that a language is described in (part of) the language itself. And before you ask, no, it's not quite the same as self-hosting (good question though). The latter is more associated with compilers that can take a source file and produce another compiler. The former is often more like an interpreter that provides support for part of a language, but those part are enough to define support for the rest of the language.
So in this case, the base implementation starts with support for only a subset of the Forth language, picked for being either essential or much more performant when implemented "natively" in the binary. A subset of this subset is then used to construct support for the rest of the language. This is very typical for Forth- and Lisp-like languages.
So in this case, the base implementation starts with support for only a subset of the Forth language, picked for being either essential or much more performant when implemented "natively" in the binary. A subset of this subset is then used to construct support for the rest of the language. This is very typical for Forth- and Lisp-like languages.
[0] https://en.wikipedia.org/wiki/Meta-circular_evaluator
[1] https://en.wikipedia.org/wiki/Self-hosting_(compilers)