Word description Declares the most recently defined word as an inline word. The optimizing compiler copies definitions of inline words when compiling calls to them.
Combinators must be inlined in order to compile with the optimizing compiler - see Combinator stack effects. For any other word, inlining is merely an optimization. Note that inlined words that can be compiled stand-alone are also, themselves, compiled by the optimizing compiler.
The non-optimizing quotation compiler ignores inlining declarations.