What is Autofill?

It means support.

If you are in queue for far too long due to your chosen roles having such a low priority, you'll eventually be given 'Fill' and be potentially placed in any other role so a match can be made. This has almost no chance of affecting you where the population is largest;

https://am-a.akamaihd.net/image?f=https://news-a.akamaihd.net/public/images/articles/2016/june/DQU/graph-autofilling.jpg?v=1

http://na.leagueoflegends.com/en/news/game-updates/features/rethinking-ranked-fives-and-tuning-dynamic-queue

at very high elos when matchmaking waits or quality risk being very poor, the system may turn on "auto-fill" which will cause every player to be eligible to fill needed roles if they have waited in queue for too long. It allows for better matches and faster matches at the expense of role selection accuracy.