Items in modules can be referenced with their full module path std::f64::consts::PI.
A simpler way is the use keyword. It allows us to specify particular items from modules we want
to use throughout our code without a full path. For instance use std::f64::consts::PI
allows me to just use the identifier PI in my main function.
std is the crate of the standard library of Rust which is full of useful data structures and functions for interacting with your operating system.
A searchable directory of crates created by the community can be found at https://crates.io.