The .git folder contains all Git repository data.
When you see the .git folder in some other folder, it means the folder containing the .git folder is versioned and contains the Git workspace.
Whenever you change any file or directory within the workspace, the Git is able to track the changes and you can submit the changes and store them to the repository at any time. When this happens, data describing the changes is stored to the .git folder in a special format.
The .git folder is hidden by default so you won't see it in File Explorer on Windows unless you changed the default configuration of File Explorer. You also won't see the folder while using dir command in command-line unless you use it with -ah parameter (which allows to display directories with hidden attributes).