What are Heroku Dynos and How are They Calculated?

Heroku is a platform as a service (PaaS) that enables developers to build, run, and operate applications entirely in the cloud. With a verified account on the free plan, you get a pool of 1000 free dyno hours per month alongside many other benefits. Your app will continue to run as long as you have some dyno hours left in your monthly pool.

Recently, I maxed out my dyno hours before the month ended and I wanted to be sure how it's calculated, may be based on traffic, bandwidth or so.

What do this dynos mean, how is the dyno hours calculated and how can I exhaust them?