The word extremely is an adverb.
Adverbs are words that modify verbs, adjectives, or other adverbs. In this case, extremely is used to intensify the degree of an adjective or another adverb. For example, in the phrase extremely happy, the adverb extremely enhances the meaning of the adjective happy, indicating a very high degree of happiness.
Understanding that extremely functions as an adverb helps in correctly using it to convey the intensity of emotions or characteristics in writing and speech.