As far as the user is concerned it's like wallet of satoshi but where the keys to the actual bitcoin are held in a multisig so most of the key holders would need to collude to steal the coins.
The fedimints would have lightning channels between one another and custodial services so they were all interoperable. Users themselves would technically have e- cash rather than actual bitcoin, but that's irrelevant as far as user experience is concerned.
Better privacy and less chance of rug pulls are the benefits over custodial. It'll also be cheaper and easier than the self custody options, allowing bitcoin to scale.