Distributed Consensus
Distributed consensus is how independent nodes agree on a single version of the ledger without a central referee.
In depth
Nodes may receive transactions in different orders or even see conflicting ones. A consensus protocol defines rules — like "follow the chain with the most accumulated work" — so that all honest nodes converge on the same history. Proof of Work and Proof of Stake are two families of such protocols.
Why it matters
Consensus is the hard problem blockchain solved; it is what lets strangers share one ledger.