I believe they mean that when you poll a future, you pass in a context. The future derives a "waker" object from this context which it can store, and use to lat

I believe they mean that when you poll a future, you pass in a context. The future derives a "waker" object from this context which it can store, and use to later trigger itself to be re-polled. By using a context with a custom "waker" implementation, you can learn which future specifically needs to be re-polled. Normally only the executor would provide the waker implementation, so you only learn which top-level future (task) needs to be re-polled, but not what specific future within that task is ready to proceed. However, some future combinators also use a custom waker so they can be more precise about which specific future within the task should be re-polled.