On the Hive blockchain, witnesses serve the role of validating signatures and timestamping transactions by including them in blocks. A block is any group of transactions (posts, votes, transfers, etc) that update the state of the database.

Witnesses are generally expected to manage a reliable block-producing node, implement a failover system, maintain a public seed node, tune blockchain operating parameters, publish correct price feeds, author/discuss improvement proposals, review code changes, and be active! Some contribute to core repositories. Some fund the development of other apps and infrastructure projects.

Every round of block production begins with the shuffling of 21 witnesses: the top 20 witnesses (by vote), plus one backup witness. Each is given a turn to produce a single block at a fixed rate of one block every 3 seconds. If a witness does not produce a block in their time slot, then that time slot is skipped, and the next witness produces the next block.

You can see the witness list here
You can see the live block production here

By voting for a witness, YOU choose which ones you trust to ensure the security of the blockchain.