What is Runtime?

Runtime provides:
Reactor: Support for performing async tasks
Executor: Support for future

Rust does not have a "built-in" runtime, but several options are available:
Runtime Desc
Tokio For writing reliable asynchronous applications. It provides async I/O, networking, scheduling, timers, and more.
Hyper Async HTTP
Tonic Async gRPC
async-std std for async