Food is a series of edible items used to feed slimes. Food is broken up into 3 (and in Slime Rancher 2, 4) main categories:
Fruit, which are found on trees.
Veggies, which are found growing in soil.
Meat, which consist of chickens, and are often found by nests.
Nectar, which can be found exclusively in the Starlight Strand and is the diet & favorite food of the Flutter Slime in Slime Rancher 2.
When a slime eats food it will produce a plort, and its agitation and hunger will both decrease. Each slime has a particular diet, and most of them will eat food from one of the three standard food categories and will not eat foods outside of its diet. In most cases, a slime's favorite food will be in the same food category as its general diet. A slime's diet can be expanded by converting it to a Largo, and can potentially have two favorite foods instead of one. A slime or largo that eats a favored food will produce double plorts.
Fruit and veggies will rot if left on the floor uneaten or if they have come near Tarr; rotten food will eventually disappear on its own, but will disappear immediately when making contact with the vac stream.
While not technically food since they do not decrease agitation or hunger in quicksilver slimes, they are considered "electricity" by the quicksilver slime's Slimepedia entry for dietary purposes. Electrical Charges can only be stored in the Augmented Vacpack.