For example, on this page: https://anime.stackexchange.com/users
I see the following:
What does this 499 mean? I can't seem to correlate it with upvotes, posts, edits or anything else.
It's the amount of reputation the user has earned in the period selected on the users page:
week | month | quarter | year | all
The default is usually "week"
It's reputation this month. You can see the explanation in the status bar of your web browser if you hover over the number.
It is the reputation for the period the page you are watching is referring: Your reputation increased of 499 in a month, and 170 in a week.