Basically, when you read a fic, in addition to giving a "kudus" you can also click this button that says "Seen". So when you are looking at a big overview list of fics, there will be a little sidebar that indicates whether you've seen the fic and whether you've given a kudos, without you having to open up the fic and try to find your name.
I read so much in so many different fandom that I find this enormously helpful! It's really nice to be able to see right away if I've read a fic or not.
Anyways, I thought it was really cool and practical, so I thought I'd pass it along. :)
ETA: I also found this IFTTT script so that you can control which notifications you get from AO3 subscriptions. Like, if you follow an author that writes in multiple fandoms, and you only really care about one of them, you can make it so that you only get an email when the author has posted something in that particular fandom.