This is the 8380th most frequent Japanese word.
怖
"怖" means "scary" or "fearful."
Here, '怖' is used as '怖い', an adjective which means 'scary'.
その映画は怖かったです。
That movie was scary.
In this sentence, '怖い' describes the noun '夢' (dream), emphasizing its frightening nature.
怖い夢を見てしまった。
I had a scary dream.
The word '怖さ' is a noun form derived from '怖い', indicating the feeling or quality of fear.