Once you have a resource checked out, you can make changes to it. If you try to make changes to resources that you don’t have checked out, a warning is displayed asking whether you want to check out the resource. This prevents you from accidentally changing resources that you don’t have checked out. You can disable this warning by unmarking the Prompt to Check Out before opening option in the Source Control tab in the Options window for Dexterity.
It’s a good idea to keep track of which resources you have checked out and modified. That way, you can check in only those resources that you made modifications to.