When the parser reads a word name, it resolves the word at parse-time, looking up the word instance in the right vocabulary and adding it to the parse tree.

Initially, only words from the syntax vocabulary are available in source files. Since most files will use words in other vocabularies, they will need to make those words available using a set of parsing words.
Syntax to control word lookup
Private words
Resolution of ambiguous word names
Word lookup errors

See also
Words