In part it means that the conference organizers have access to the conference administration tools and resources that IEEE, ACM, or the like provide. Having been around some IEEE-sponsored conferences, that can mean things like payment processing and credit card handling; access to a journal special issue for proceedings; conference organization website for registration, submissions, and reviewing; and registration discounts for members of the society.
Access to these features doesn't come for free. The professional societies charge for their use and stand to make money on them. As such, while you have to propose the conference and get it approved, after that the oversight is less. The quality of a conference comes down to the Organizing Committee and it's technical/content/papers committee. If you've never heard of any of the members of these committees, or many of them appear to have low-quality papers, there's a good chance that the conference isn't all that great.