RPC stands for remote procedure call; it is a method of executing a function or method on a remote machine. RPC has been around since the dawn of time and there are many different types of RPC technology some of which relies on there being an interface definition (SOAP, Thrift Protocol Buffers). This interface definition can make it easier to generate client and server stubs for different technology stacks. Generally, the interface is defined using a DSL (domain specific language) and a generator program will use this to create application clients and servers.
Where REST needs to use HTTP as a transport layer, RPC is not bound by this constraint, and while it is possible to send RPC calls over HTTP, you can use the lightness of TCP or even UDP sockets if you choose to.
RPC has seen a resurgence in use lately with many large-scale systems built by the likes of Uber, Google...
