"Food" can be countable ("a food", "many foods") or uncountable ("some food", "a lot of food"). The countable sense is used to refer to a particular type of food, or, in the plural, several different types. The uncountable sense means food considered as a general "substance", without particular regard to the varieties it can come in.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.