Witnesses are the 'miners' / block producers of the Delegated Proof of Stake networks (Peerplays, Steem, Bitshares, YOYOW).

Witnesses are elected into their position by Stakeholders votes, at a minimum; they are responsible for validating transaction, compiling transactions into blocks, signing them and broadcasting them to the network.

For example this comment is considered a transaction; it will be broadcast to the network and seen by the next 'witness' who will validate the transaction and include it in their 'block', once signed it will be sent to the rest of the network and be seen by the other witnesses.

You can learn more HERE