Upvotes direct money from the reward pool toward posts. Essentially, the people that are producing steem coins (miners) divide them up between content creation pools and curation reward pools. When you upvote content, you're signalling to the system that it should direct some of that creation reward pool to that post. The same goes for curation. You don't actually pay out anything from what you have toward your upvotes; it's paid via the reward pools.