Items in the food
First foods are generally composed of components of a single item, like green beans, with not fillers.
First foods help you identify food reactions like vomiting
& rashes. If no problems show in 0-4 days, the item is a "safe" food. Second foods will have components of 2 foods. Feed more than 1 new thing at a time & you won't know what caused the rxn. I've hospitalized several with rxns.