A STEEM witness is a person who operates a witness server (which produces blocks), and publishes a price feed of STEEM/USD to the network which is how STEEM Dollars are always worth 1 US Dollar in STEEM (using the 7 day exchange).
These are the witnesses I voted for:
RE: Show Us Your Witnesses Weekly Contest: A Concept To Drive Up Witness Voting Engagement Among Steemians (Episode #4 Result Update; Episode #5 — 6SBD Prize Pool; 3 Possible Winners!)