Back in the dark ages (1984), there were precious few ways to write software for the Mac on the Mac. Apple wanted you to develop on a Lisa. Mac Pascal was an interpreter. Mac Basic was killed by Microsoft. I bought one of the few available options - MacForth. As I recall, it's a decent implementation with words defined for calls into the toolbox. It came in several versions with different costs. I think I paid $75 or $125. The professional version(s) were quite expensive. I think it was fairly popular because it was the first system that let you write stand-alone applications on the Mac. It even ran on a 128k.
IMO Forth is a quirky language, but highly interactive. This interactivity can make it very productive for developers. Forth is a stack-based postfix language. I tried, but I never learned to think in Forth. I did write a very simple terminal emulator, but I can't recall much else.
There are several free Forth systems for the Mac. Check out forth.org for more info, including tutorials, primers, and online books.