my boyfriend just started doing meal prep and I'm thinking of doing it too, can anyone give me a kickstart on how to do it properly?

he's been doing it for a couple of days now, he has a recipe book and it just has a whole bunch of stuff in there that you can make for the week, Ive heard of people doing meal prep though, but I'm not actually sure how it all works. He's mainly doing it for weight loss. Im trying to do it for my allergies. What do people typically do?