Witnesses earn STEEM POWER by running witness nodes which is something like mining, but not so computational intensive. This is called staking and is used in Proof of Stake based blockchain when one is able to get rewarded based on its actual stake. Steem as a blockchain is a little bit different in this case, because the amount of stake that you are able to mine for does not depend only of how much stake you currently have, but it depends also of how much stake the people who have voted for you as a witness have.
If you look on steemd on some witnesses accounts, you will see the producer reward transactions. This is how they actually make money, and this is also generated from the same reward pool you are payed when you claim your rewards :)
Thanks! I see, the pos reward varies with our votes due to our stake, I ll have to chose my vote with care :)
Peace & nice answer!