The .git directory is the git repository directory which is where almost everything that Git stores and manipulates is located.
GIT_DIR is an environment variable that you may set to locate it.
More information on the dedicated page: Git - Repository (Directory)
| Subdirectory | Entity |
|---|---|
| objects | the database objects (commit, file, directory) |
| refs | the refs (named hash) |
| hooks | the hooks 1) |
| info | configuration |