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Witnesses are Steem users who run Steem full nodes that produce blocks and keep the network running 24/7. In return, they receive a good amount of rewards for that. The witness that has the highest share of votes receives the highest amount of rewards.
Since receiving more votes increases their earning, they actively campaign for getting more Steem users to vote for them. The power of the votes they receive depend on the Steem power of the user who voted for them.
Witnesses are kind of similar to elected politicians in the sense that we elect them to govern the Steem blockchain. Witnesses also provide the feed price to control SBD printing rates to ensure that SBD doesn't go below 1$.