Main thing I've learnt is testing. Automated testing, manual testing. Exploratory, checking etc.
In University and my previous freelance work we didn't do much at all. So joining a company and seeing how this is done was really interesting to me.
One thing that was especially new was the idea that testing is owned by everyone including developers. If you feel like you're the closest and most valuable to test then you should. I think that's interesting view that is against the QA approach.
I'm still learning but feel so much more comfortable with it now, think it's an interesting area.