Please explain how to understand that the server on Heroku "falls asleep"?

Question

Heroku

I can't figure out what the point of this statement is.
And what should he do if no one uses the server?
Can you explain with an example?

Answer 1

After some time, if there are no requests to the web application, its process is forcibly killed. So Heroku save machine resources.
But if a request comes later, the web application starts again. There will be some delay.