It seems to be a non-common word comparison score, which is often mistaken.
Basically, when a post is submitted, the system creates a word list for the post, strips the common words out of it, then creates an association between the post ID and each remaining word. When a search occurs, each word of the search phrase is checked against the db holding the associations and the post with the most matching associations will be considered the most relevant.
It can be seen how this system could be incredibly incorrect when matching posts to a search phrase but it's really the one that's most often used in software, due to it's ease of implementation and low overhead, resource-wise.