Let's say we are analyzing posts to a sports forum, where the records contain a title, a body (text) field, and keyword fields, where the keywords might be the name of the sport that the post concerns.
I know how to do aggregations around keywords, so that I could report on all the posts that have the keyword 'tennis'.
But let's say that I want to do queries in the same manner but the term is inside the text body field, not in a keyword. So e.g. I want to aggregate on every post that contains the word 'exercise' somewhere in the body field, just as though that was a keyword field even though it isn't.