The FORTH-I/88 ROM chip is an 8K module providing standard Forth-79, a programming language preferred by higher-grade programmers and the mathematic community because of its speed improvements over BASIC. It's syntax is Reverse Polish Notation, which was used by a lot of scientific calculators of the day. From what I can remember it's stack based, so instead of the standard BASIC command PRINT 2+2 you put the numbers on the stack and then tell the interpreter what you want to do with them, so the equivalent FORTH command would be "2 2 +". There was only ever 1 home machine to use FORTH natively, and unfortunately it didn't sell well at all. It was the Jupiter Ace.

The bumf for this one states:
Forth-79 standard
Include line editor
Provide RAM DISK Feature
Maximum 40K User RAM support.