Git is a program that keeps a project's history: every version is recorded with what changed, who did it and when, and you can go back. A repository is the project folder together with all that history. GitHub is a site where a copy is stored online.
A question from the app. Pick an answer.
Two agents work at once in the same repo and edit the same files. What do you do?
Each one edits its own working copy of the same repo, without stepping on the other. Merging the changes can still bring conflicts.
Tests do not separate the files: both keep editing the same copy, one on top of the other.
Slow or fast, they edit the same files. They just take longer to collide.
Think of a shared document with tracked changes, but for the whole project. When something is in good shape, you save a "commit": a snapshot of the project with a message, like "add the pay button". Commits chain together into the history. The repository is that folder with its chain of commits.
Git lives on your computer and works without internet. GitHub is something else: a service that stores a copy of the repository so you can back it up and share it with your team. There are alternatives, like GitLab or Bitbucket. That is why people say "I pushed it to GitHub", but git is what keeps the history.
A coding agent works on a repository and can touch dozens of files in minutes. With git you get two lifelines. One: going back. If the agent broke your store's checkout, you return to the last good commit instead of repairing blindly. Two: seeing what changed, with a diff, instead of trusting only its summary.
A simple habit: before asking for something big, save a commit with what already works. That way the worst case is going back to that point. And if your project has rules for the agent, like those in a CLAUDE.md, they travel inside the repository, just like the code. For two agents to work without stepping on each other, git lets you give each one a separate working folder, called a worktree.
"Git and GitHub are the same thing." No: git is the tool and GitHub is a place to keep repositories. "If I did not commit, I can still go back." Not always: git only remembers what you saved, and an uncommitted change can be lost if someone overwrites it.
And one warning: the history remembers everything. If an API key ever landed in a commit, deleting it later does not remove it from the past, and in a public repository anyone can see it. Revoke the key and create a new one; do not count on hiding it.