git
Git is a tool that remembers every version of your code, so you can save your work, look back at what changed, and undo a mistake without losing everything. Think of it as an unlimited 'undo' history for an entire project, with a note on every save explaining what you did.
It's distributed, which is a fancy way of saying every copy of the project is complete: your laptop has the full history, your teammate has the full history, and the server has the full history. You can work, save, and browse the past even with no internet.
One thing to keep straight: git is the tool, GitHub is a website that hosts your git projects online. You can use git all day without ever touching GitHub — they're not the same thing.
$ git status On branch main nothing to commit, working tree clean
Ask git where things stand: here, nothing's changed since your last save.
Created by Linus Torvalds in 2005 to manage Linux. The name? He cheekily called it 'the stupid content tracker'.