Sure, happy to. I'm copy/pasting info that I had used in another post on witnesses :) Let me know if you have further questions, and I'm happy to answer!

• The primary duty of a witness is to produce and sign blocks for the Steem blockchain. This is the way that everything happening in Steemit gets accurately recorded and tracked on the database that is distributed across all the Steem nodes.
• Witnesses are also elected leaders of the community. They have a responsibility to grow the Steem/Steemit community by using their skills/talents (as well as witness funds) in ways that benefit the community.
• They are the final deciders as to whether or not hard-forks occur. If there are controversial or bad changes being proposed, the witnesses can put a stop to them by refusing to pick up the new version.
• They also publish the price of Steem, which is used to determine the payout of SBD->Steem conversions, as well as the Steem marketcap - which determines the total amount of rewards available in the reward pool.