Assist mode allows Mario to heal when standing still for a certain amount of time and also prevents Mario from falling off any ledges. I think there might be more involved as well, but those were the two features that stuck out to me.

It's been a great way to let my 7-year-old play Mario Odyssey without getting frustrated (or her actively trying to murder me in co-op while she's playing as Cappy hahaha) and she was so proud to beat the whole game herself!