What does it mean when broccoli flowers?

I couldn't find broccoli seedlings anywhere, so I planted it from seed - much too late. A few plants are doing well -- at least I think they're doing well. What does it mean when broccoli develops flowers? Is it overripe? Is it ripening? I haven't had much success with broccoli (or cauliflower, either) in the past, so this is all new.

It's a lovely sunny day, and it was hard to isolate the broccoli in a decent picture. Later tonight I'll get at the squash plants (powder mildew, must prune), and I'll stare at the broccoli and wonder what on earth happened here - or if perhaps it's just growing the way it's supposed to grow. Gardening, after all, is a learning process.