From studiobinder: Ultimate Guide to Film Terms…:
Flag
A Flag is a black, light-absorbing cloth (duvetine) stretched on a metal frame and used to block out areas of light in all different sizes.
Barn Doors
Barn doors is slang for the four metal folding doors found on all sides of a light. The barn doors can be repositioned to help direct light in a certain direction.
RLM is usually really good about explaining film industry concepts and jargon so I was surprised they didn’t clarify those. Enjoy your Blu-ray!