We put an adjective in front of a noun to describe the noun.
Tom has a big house.
Tom has a beautiful house.
The adjective big and the adjective beautiful both describe the noun house.
Instead of writing two sentences about Tom’s house, we can put both adjectives into one sentence, like this: Tom has a big and beautiful house.
We can also replace the word and with a comma, like this: Tom has a big, beautiful house.
Here are more sentences with two adjectives joined by a comma:
Mary is dating a tall, handsome man.
Danny handed me a cold, wet fish.
I like sitting in the quiet, peaceful woods.
We watched a long, scary movie.
If we use two adjectives to describe a noun, we put a comma between the two adjectives.