Each block of transactions contains a fingerprint (hash) of the block before it, forming a chain back to 2009. Changing any old transaction would change its block’s hash, breaking every block after it — an attacker would have to redo all that proof-of-work faster than the honest network extends it.
Anyone can download and verify the whole chain by running a node; that radical auditability is what replaces trust in a central bookkeeper. "Blockchain" as a standalone buzzword borrowed the credibility of this design — usually without the decentralization that makes it meaningful.