How Free Heroku really works and how to get maximum from it?

Recently I’ve got a problem that Heroku stopped my sites (I’ve got 6) because I’ve had used all free dyno 1000 hours/month

It was very confusing for me WTF does this free dyno hours mean?

I couldn’t figure out it from the docs so after speaking for a couple of hours with the support I finally got it and want to share how to get maximum from Free Heroku accounts.