It is the annual return on your hive power. If you have 100 hive power and your apr is 3%, then it means that in a year you will earn about 3 hive in curation rewards.
If you get that up to 10%, it means you get 10 hive.
I’ve seen people with 20%+ apr on hive. Mostly bigger accounts that can do lots of small votes, but that are careful to always vote with enough hive power to make sure they get a return on their investment.