"What is it, garlic?"
Italian does use the definite article in many situations where English doesn't. Most of those times it's actually more correct to use the definite article than do an word-by-word translation from English... But in this case I don't know enough to be sure either way.
This is the most useful link about the definite article I've found (below) and while it still doesn't give me enough to be sure, I'm going to guess that you don't use the article because in this case you're not referring to garlic in the general collective sense or as a category. (Instead you are pointing at a particular object and saying "what is it?")
But if a native speaker happens to weigh in on this discussion, take their word over mine...