Very simply put - Git is a version control system so that you and your teammates could work together efficiently on the same codebase. When I make a change to my code, I can upload it to a remote server where my teammates can track what changes I've made.
Github is just one of the most popular Git service providers.
A very straightforward, no BS tutorial I find useful can be found here:
It's more about how than what.
To understand what is Git and how it works, I found this video to be useful:
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