Ah! So the steem blockchain, like many blockchains, requires computers to confirm and record changes to the blockchain. A witness node performs that computation. A witness is also a person who oversees that process. With steem, I believe posts you make here, upvotes, comments and so on are written into the blockchain. For more information, see https://steem.io/SteemWhitePaper.pdf or search steemit for "steem witness"