Countable nouns are the names of objects, people, etc. that can be counted. For instance, book, toffee, shop, orange etc. They have plural forms such as books, pens, oranges etc.
Uncountable nouns are the names of things which cannot be counted. For example, greed, milk, oil, water, air, silver etc. They do not have plural forms and thus we can’t write oils, airs, silvers etc.