The Field Guide was where players could look to find the items they found during quests (finds section), bits they owned or had owned (Stuff section), Power Ups that they had the ability to craft (Power Ups section), gadgets they had crafted (Gadgets section), Kart Parts they owned (Kart Parts section), and, as of the May 2018 update, Racing Gadgets.
It could be accessed from the Craft It Booth or while crafting anything from the crafting menu on top of the screen. Your friends could also access yours.
It was the only place in the game where the names of the Quest Worlds were listed by name.