I didn't find it even the least bit confusing. It seems pretty obvious to me what it means. "You're guaranteed one of your selected roles this game because you recently autofilled." So you are guaranteed to get one of the roles you selected this time, and cannot be autofilled. Is the confusion is coming from the fact that you can gain this by manually selecting fill yourself? It was probably done so to try and offer up a little bit of something for people to queue fill, even though it really doesn't matter.
I responded because the only way this wouldn't make sense is if you didn't know what Autofill is. To clarify, Autofill is a feature Riot implemented for times when queue lengths are excessive. If it triggers, it will then take people outside of their selected roles and push them into a match just to get a match going. Example : You queue Mid/Top, queue times are deemed too excessive, autofill kicks in and puts you into a match as support. You will gain this autofill immunity for your next match. You can also gain this protection by manually queueing fill, but that isn't terribly meaningful because someone who manually queues fill probably already doesn't mind being filled.
Does that help?