What @mycoft said :)

As you post, comment and upvote articles you could earn Steem or Steem power. You also have voting power, although I'm not sure if you can directly see your current voting power on Steemit.

If you go here and type in your username you can see your voting power:

https://steemstats.com/#/

Basically, when someone upvotes a post or a comment (or maybe just a post, I can't recall) they lose a little of their voting power. Eventually their voting power gets replenished if they don't continuously don't "spamvote".

When they upvote, the value of that upvote (to my knowledge) is determined by their voting power and the amount of Steem Power they currently have.

I don't know what the specific formula is, though.