Start with the idea
Calling an async fn does nothing on its own: it builds a state machine that makes progress only when polled. .await yields at that point, letting the executor run something else while the operation is pending — which is how one thread can service thousands of waits.
Calling an async fn creates a future that represents work to be performed. It does not run the body to completion immediately. An executor polls the future; it may return Poll::Pending, register a way to be woken, and yield the thread to other tasks. When the awaited operation can make progress, the waker schedules another poll. Eventually the future returns Poll::Ready(value). This is why many I/O waits can share a small number of threads.
An .await marks a point where the task may suspend.
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