Basically three long endurance tests, each made up of a series of challenge rooms. Strong enemies, no gear. It tends to focus on different gameplay themes every few rooms which is neat, but overall the trials are kind of... I don't want to say underwhelming because they're exactly what I thought they would be, but it's just kind of alright. (Hoping DLC 2 is significantly more substantial.) But it did remind me of how much I loved all the different systems in the game interacting with each other, and I spent a bit of time just running around the world for the hell of it.
(Also: If you haven't finished it yet, try to find all the Memories before going for the ending.)