I keep seeing this 'Diet' thing on the bottom left of my beast's profile and I wonder what it's about. I have the things that are listed, but how do I use them?

Newbie here; x_x;;

Diet composition affects how stat points are allocated each time a pet gains a level. Hover over the ? to see what food corresponds to what characteristic. Click on the stars to change how much of each food the pet will get.

You don't need to do anything to feed your pets; they automatically get fed at the beginning of each day.