A countable noun is a common noun that has singular and plural forms and can be modified by a number.
The opposite of a common noun is called a mass noun, which does not have different singular and plural forms, nor can it be modified by a number.
As a general rule, words referring to objects and people are countable nouns, and words referring to liquids (water, juice), powders (sugar, sand), and substances (metal, wood) are mass nouns.
When you travel you have two suitcases. Suitcases are the same as luggage, but you cannot say "two luggages" as luggage is a mass noun. When you travel you have luggage, or two pieces of luggage. Mass nouns use measure words like pieces of to make plurals.
You want to build a bookshelf so you buy eight boards made of wood. "Wood" is a mass noun, so it is incorrect to say you have "eight woods," but you can say you have eight pieces of wood.
Here is a list of some more mass nouns: advice, air, art, blood, butter, data, deodorant, equipment, evidence, food, furniture, garbage, graffiti, grass, homework, housework, information, knowledge, mathematics, meat, milk, money, music, notation, paper, pollution, progress, sand, soap, software, sugar, traffic, transportation, travel, trash, water
There are some words that are both countable nouns and mass nouns. You leave some papers on the desk, by which you mean you leave some specific documents. If you leave some paper on the desk, you mean you left a package of paper or just some paper in a general sense.
Search for some mass nouns on Yabla English and see how they are used in context.