No, that's a great question!

@pfunk put together an excellent guide here explaining what a witness is: https://steemit.com/steemit-guides/@pfunk/a-full-steemit-user-s-guide-to-steem-witnesses

Essentially witnesses create new blocks for the Steemit blockchain and ensure the system continues to run smoothly. They make decisions about future code changes to Steemit as well as parameters which impact the economics and signup fees.

In other blockchain systems, the miners have all the control and sometimes they don't act in the best interests of the users. Also, they can't be voted out of their position of influence. The Steem blockchain is different and uses delegated proof of stake which @dantheman recently explained in this white paper. Witnesses are part of that system and they can only hold that position if the network votes to give them that role.