Adverbs modify both verbs and adjectives, but adverbs can also modify other adverbs.
Adverbs that modify other adverbs are called Intensifiers. An adverb intensifies another adverb.
Look at this sentence: Alex walked slowly down the hill.
The word slowly is an adverb modifying the verb walked.
Now we can add an intensifying adverb to tell how slowly Alex walked.
Alex walked extremely slowly down the hill.
Alex walked very slowly down the hill.
Alex walked somewhat slowly down the hill.
The adverbs extremely, very, and somewhat all modify the adverb slowly.
Here is another sentence: Grandpa always rests before dinner.
The word always is an adverb modifying the verb rests.
Now we can add an intensifying adverb to modify the adverb always.
Grandpa almost always rests before dinner.
Grandpa nearly always rests before dinner.
The adverbs almost and nearly both modify the adverb always.
Adverbs that modify other adverbs are called intensifiers because they intensify the adverb modifying the verb.