In order for a software application to interact with the Ethereum blockchain - either by reading blockchain data or sending transactions to the network - it must connect to an Ethereum node.
For this purpose, every Ethereum client implements a JSON-RPC specification, so there is a uniform set of methods that applications can rely on regardless of the specific node or client implementation.
JSON-RPC is a stateless, light-weight remote procedure call (RPC) protocol. It defines several data structures and the rules around their processing. It is transport agnostic in that the concepts can be used within the same process, over sockets, over HTTP, or in many various message passing environments. It uses JSON (RFC 4627) as data format.
4EVERLAND offers RPC services for most mainstream chains to facilitate Web3 developers, eliminating the need for running their own clients and enabling interaction with various chains through API requests.