If something is countable, it means that you can count it. You can use "a" or a plural. When a thing is countable, it changes words you use in your sentence, like "much" or "many" and "less" or "fewer". It's that simple. But knowing if something is countable isn't always easy.
Countable nouns are always about the same shape or size. It is easy to think of "one" or "a". They have a limit or edge to their shape, like trees, cats and planes:
It is easy to tell if you have one tree, two cats or three planes. These things have a clear shape. There is a limit to the idea of 'a cat'.
Another way to know if something is countable is if you and someone else think about a countable thing, they will be about the same:
When my friend and I both think about a flower, there is no confusion. Out ideas are more or less the same.
Lastly, if you can draw a picture of "one" then the thing is countable:
Uncountable things can be many different shapes and sizes, and thinking of "one" or "a" is hard:
"Water" is uncountable because there is no clear shape or limit of one. Is "one water" a drop of water, a glass of water, a pool, a wave, a river, a lake, an ocean, rain, a sea...?
The key is that my idea of "one water" may easily be different to your idea of "one water". When we think about 'one' water, this might happen:
I might think about a wave, but you might think about a drop. These are not at all the same!
Groups and Members
Often the name of a group of things is uncountable and the members of that group are countable: