In grammar, a countable noun is a noun you can count: one book, two books. It has a singular form and a plural form. You can put a or an in front of it, and you can put a number in front of it. In learner dictionaries, countable nouns are marked [C]. Apple, student, and idea are countable. Water, air, and advice are not — those are uncountable, marked [U]. Countable is a classroom word more than a street word: teachers and dictionaries say it; people in shops do not. Though actually, English is messy here. You can say “two coffees” in a café, stretching a word that is often uncountable. The tidy dictionary label is still useful, as long as you know real talk sometimes bends it.