"..." is not a Forth word, it is used here to indicate an arbitrary sequence of Forth words.
However, it could be a Forth word, as "..." is a permissible name. The rules for naming Forth words are;
This permissive attitude is typical of Forth. It has very few rules, but lots of conventions. One that we have seen already is using ."dot" to mean "print". So ." means print something. In this instance print a string literal delimited by the following " (Another Forth convention).
As an example, we could devise a program that explains "Smileys" when the user types them at the command line;
: :-) ." I'm a happy face, indicating a light hearted comment." ; : :-( ." I'm a sad face, indicating displeasure." ; : [:-|] ." I'm Frankenstein's monster." ; : >+++:) ." I'm a skeletal fish, indicating something fishy here." ;
And so on...