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Github is for code collaboration. You can make commits to repositories which means that you can add new stuff to existing code. You can also report issues and make contributions.There is a "hello world" guide on their site to start with.
pycharm let's you use git for version control (which is github). What that means is, it allows you to document and save your progress as you code. Not just the immediate, but any previous versions. It's useful because you can safely delete something, knowing that you can just go back a patch. It also helps you keep track of what changes you made. This can be seriously important as tiny changes can be overlooked or even forgotten. This is life saving if your code gets into the hundreds of lines or higher. I've had times where I'm like (this code is a disaster, I'm gonna start from scratch), delete it all, but then want to know how I solved an issue by looking at my older code.