This is the 116th most frequent Korean word.
너무
"너무" means "too" or "very" in English, depending on context.
In this sentence, '너무' is used as an adverb to express a higher degree of intensity of the adjective 'hot'.
너무 더워서 밖에 나가기 힘들어요.
It's so hot that it's hard to go outside.
Here, '너무' emphasizes the adjective 'interesting' to denote an extreme level of interest.
그 책은 너무 재미있었어요.
That book was so interesting.
In this case, '너무' conveys excessiveness regarding the adverb 'quickly', highlighting the undesired speed of speech.