Countable Nouns are nouns which can be counted and made plural, e.g. 1 dog, 2 dogs. They are also known as Count Nouns.
On the other hand, Non-Countable Nouns can’t be counted or made plural, e.g. water, rice, air, etc. They are also known as Non-Count or Mass Nouns.
For more, see Count and Non-Count Nouns in English Grammar.