As explained in the comments of the code, calling std::sync::mpsc::channel() generates a tuple consisting of a Sender and a Receiver, which are conventionally called tx for transmission and rx for reception [12].
This naming convention doesn't come from Rust, but has been a standard in the telecommunications industry since at least 1960 when the RS-232 (Recommended Standard 232) was introduced, detailing how computers and modems should communicate with each other.
These two halves of the same channel can communicate with each other independently of the current thread they're in. The module's name, mspc, tells us that this channel is a Multi-producer, single-consumer channel, which means that we can clone our sender as many times as we want. We can use this fact to our advantage when dealing with closures [16 to 21]:
for i in 0..10 {
let tx = tx.clone();
thread::spawn(move || {
println!("sending: {}", i);
tx.send...