From what I have learned,
New coins are created by Steemit (the company) and they are left in a kitty.
When you 'up-vote' someone, it is taken from this Kitty (and NOT from you).

You are basically helping to 'share' Steemit's Kitty with deserving people.

PS:
If your 'reputation' (the numerical score after everybody's profile name) is low, then it will allocate a lower amount of Steemit dollars compared with someone who has a much higher reputation.
ie: the higher your 'reputation' score, the higher the amount allocated for the up-vote.

When you up-vote, you are helping to spread the value amount the community.. (at no cost to yourself) :)

So, go ahead and up-vote some people :)