A key word list in WordSmith Tools looks something like this.
The key words are words which occur unusually frequently in comparison with some kind of reference corpus.
Beside each key word there are various numbers telling you how frequent each one was in the source text(s) and how that compares with its frequency in the reference corpus.
In the list above, based on 8 sections of the play Romeo and Juliet in comparison with the British National Corpus, we see lots of names of the main characters, some archaic words like thou and thee, and love.