RPC or remote procedure call is a request-response protocol designed to allow a program on one computer to call a subroutine (standardized procedure or operation) on another machine. In general, it looks like Alice is asking Bob to do something without Alice knowing that Bob is remote.
In RPC protocol, each call (request) is matched by its answer (response). So, there’s no need for developers to write the same programs repeatedly: it streamlines the development process and makes the use of computation resources more cost-efficient.
The RPC Public Service can be used in several situations. For example, you can check a wallet balance, a transaction, or a validator. You can check almost any information from the blockchain. An RPC is basically a blockchain node that does not validate, but sends / receives data from / to the blockchain. It can also help with transactions on the chain.
Easy2Stake has also set up some servers used as RPCs and offers them publicly.