Rainbow is an informal game term. A rainbow deck runs cards from many different cultures, rather than just focusing on one or two per side. This means it can splash lots of strong cards from different cultures. But it can't easily run cards with strong cultural enforcement, and it's vulnerable to Rainbow hate cards like Gríma, Chief Counselor (5R51).
Rainbow decks are most common in Fellowship Block, which has little cultural enforcement and no cards like Grima. Rainbow decks do pop up in later formats, though, with decks like Rainbow Wounding in Movie Block and sgtdraino's Sudden Strike deck in Expanded.