There is some info here: https://steemit.com/witness-category/@timcliff/witness-voting-guide

By voting on witnesses, you are helping to decide who powers and secures the blockchain. The witnesses themselves are paid by the blockchain for the work they do.

The more votes witnesses receive (measured in the Steem Power of the voters who are voting for them), the higher they are ranked. Higher ranked witnesses produce more blocks, and are paid more.