You have 30 votes that you can give and take whenever you want. When you vote for a witness, you give that witness support, so that their voice on decisions that affect all steem is heard louder.
The top 50 witnesses can be voted immediately on the page, all other witnesses (like me f.e.) need to be inserted with username to vote for them.
A witness earns some steem every time they produce a block. And they get to produce more blocks the more you vote for them. So your vote decides which witnesses get to earn your share of the witness pay.