Adjectives modify nouns or pronouns (What a pretty house! She is tall).
Adverbs modify verbs (She types quickly), adjectives (She is extremely tall) or other adverbs (Please type more quickly).
When an adverb modifies an adjective (e.g. “she is extremely tall,” no hyphen is necessary. I see many authors make this error in their book descriptions and personal bios. For example:
- The world in this story is inhabited by fully-functional robots that act like humans (INCORRECT)
- The tale takes place on a currently-active landfill (INCORRECT)
- When he’s not writing books, John works as a highly-trained specialist managing labor disputes (INCORRECT)