A relayer is an off-chain service that forwards messages or transactions between systems, most often in bridges, rollups, and other cross-chain setups. It watches one network, packages a signed message, and submits it to another network or application. The relayer is not the blockchain itself; it is the transport and delivery layer around it.
Relayers matter because they sit on a critical trust path. If a relayer is compromised, misconfigured, or fed bad state, it may forward fraudulent messages, trigger incorrect asset releases, or stall transfers entirely. In attacks on bridge systems, weakness in relayers, validators, or admin tooling can be enough to create a drain even when the core contract looks intact. Defenders harden relayers with signature checks, redundancy, strict access control, monitoring, and replay protection so that a single off-chain component cannot become the weak link.