Consensus protocols are a solution to the double-spend problem: the challenge of preventing someone from successfully spending the same digital money twice. The hardest part about this problem is putting transactions in order: without a central authority, it can be difficult to resolve disputes about which transaction comes first when you have two or more mutually-exclusive transactions sent around the same time. For a detailed analysis of the double-spend problem, how the XRP Ledger Consensus Protocol solves this problem, and the tradeoffs and limitations involved, see Consensus Principles and Rules.