Countable nouns are people, places, and things that can be counted.
two cats, five fingers
Non-count nouns usually cannot be counted and lack a plural form. They do not require an indefinite article (a/an). Also, they take a singular verb.
Knowledge is power.
Cheese is delicious.
The main issue writers will encounter with count vs. non-countable nouns is adjective use, specifically when using the following:
“Many” vs. “Much”
Use “many” for count nouns and “much” for non-count nouns.
I ate too many cookies.
I drank too much milk.
“A Few” vs. “A Little”
Use “a few” for count nouns and “a little” for non-count nouns.
I printed a few copies.
I used a little ink.