In Forth, it is customary to explain a word's action by drawing the top of the stack before and after the word's execution. The two halves are separated by `--'. For instance,
/MOD ( n1 n2 -- mod quot)
means that
/MOD takes
2 numbers and returns `mod' (presumably
n1 MOD n2, although the stack diagram only implies it if you know some
conventions) and `quot' (presumably
n1 DIV n2).
The space after the open-parentheses is important: it makes the entire stack diagram a legal Forth comment. Stack diagrams are often written in just this format at the definition of the word:
: ABS ( n -- u)
dup < if negate then ;