“Cool” is a very useful and common word in English. It has various meanings.
One meaning of cool is カッコイイ. For example,
I saw a very cool car yesterday parked at the mall.
My brother does not look cool wearing a hat.
Another meaning for cool is to be relaxed and calm. For example
The policeman was able to stay cool during the robbery.
I couldn’t keep my cool during the test so maybe I didn’t get a good score.
Cool can also mean not too warm and not too cold. A comfortable feeling of a little cold. For example,
The weather recently is very cool. I love the cool nights in autumn. The mornings in fall are also nice and cool.