When you upvote someone's post, or your own post for that matter, you are rewarding them with Steem Power (SP) and Steem Dollars (SBD) that will be newly created at the next scheduled payout. The amount of the reward your upvote is worth is based on the amount of SP you have.
For example, I have about 570 SP and when I upvoted your post just now, your reward jumped to $0.02 because of my SP. As you get more SP, your votes will be worth a bigger reward of newly created SP and SBD for posts you upvote.
@gregory-f: I liked your reply and tried to up-vote you. I have a huge amount of Steem Power and when I upvote people it is usually worth a couple of dollars to them. However, your balance didn't move. Is the reason:
Number 1 is mostly correct. You cannot give a reward to a post or comment that is over 7 days old. Actually the cut off for a reward is 12 hours before the 7 day mark.
Number 2 is a little correct, in that you have a limited amount of rewards available and every full power upvote reduces your voting power by 2%. You also recharge voting power by 2% every 2.4 hours. So if you only give out 10 full power upvotes every day you will stay close to full power.
You can see your voting power level here: https://steemd.com/@swissclive
You've given the best explanation i've read on this post. It's just simple.
Thank you!