Countable nouns are for things we can count using numbers. They have a singular and a plural form.
Look at the picture above. How many apples can you see? How many bananas? How many tomatoes? Let's count! Five apples, four bananas and one tomato.
In these examples the nouns apple, apples, banana, bananas, and tomato, tomatoes are all countable nouns.
Uncountable nouns are for the things that we cannot count with numbers. They have no plural form.
Look at the picture above. There is some bread in the basket, cheese on the plate and milk in the jug. We can't count bread itself, but we can count slices of bread. We can't count cheese, but we can count pieces of cheese. We can't count milk, but we can count jugs of milk.