eth-lsd-relay is an off-chain relay service responsible for interacting with Eth Lsd contracts and Beacon chain, synchronizing blocks and events, handling tasks related to validators and balances, and other off-chain operations. Each voter is required to run an instance of this service to submit their votes for new balances of the network, ejected validators and rewards distribution etc.
To learn more about ETH LSD stack, see ETH LSD Stack Documentation and Guide