When you check out an object, the object locks so that other users can't make changes to it. When you check in an object, a new version of the object is created in the source control repository.

If you check in an object that's in a folder or project that isn't source controlled, the folder or project becomes source controlled. An asset can't reside in a source control repository unless it's in a container such as a project or folder.

If you import assets into an organization that uses source control, the assets must be checked out so that they can be overwritten.