See the comments below: it is a measure of reputation based on votes your posts receive. You start in the 20s (they rescaled it since this post). If you receive upvotes from higher rep users it increases your score. Down votes decrease it. Spammers with lots of downvotes and negative rep have posts hidden by default.

In other news, I have now been on Steemit for a year. This was me learning the system. There's lots to learn, but people are generally quite helpful.