If you use a defined article (die, der , das) you mean something specific. If you put an undefined article before a noun you are (mostly) referring to one out of multiple where you don't care which one. If you use no article, it can be both, but most of the time it's the same as with undefined article
Wir brauchen Brot und Käse
refers in this context to some bread and some cheese in general. In English this would be We need some bread and cheese
Wir brauchen das Brot und den Käse
is referring to a specific item: There is bread and cheese over there on the counter, can you get it?
Wir brauchen ein Brot und einen Käse
refers to some unknown bread and cheese: I don't know where bread and cheese is, but we need some
In plural you can't use 'ein', so you only have defined article (specified) and no article (unspecified)
Wo sind Tomaten
refers to some tomatoes in general, for example in a supermarket: Where can I find tomatoes?
Wo sind die Tomaten?
refers to some tomatoes you may have bought earlier: Do you know where I put the tomatoes?