Check out an object so you can make changes to it. Check in the object when you want to update the source control repository with your changes. You can also check in objects to add them to the source control repository.
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.