The patent describes a process by which iOS would highlight a word that you've typed that had been autocorrected. Like many other systems will underline a misspelled word with a red line, this would place a blue line under a word that has been autocorrected. The idea is obvious -- it would theoretically bring more attention to autocorrections and maybe reduce the number of times autocorrect fails happen.
Of course, in practice, it seems unlikely that such a system would really put a dent into all of the fun that we have with autocorrect fails. The main reason that autocorrect fails happen is because people send messages quickly without reviewing the text. Adding a blue line under a word doesn't seem like it would do much to fix that issue.
We'll have to see when Apple actually introduces the feature though.