The -ly at the end of an adverb is called a suffix or the trailing part of a word.
Like adjectives, adverbs are used to modify another word, in this case the action portion of a sentence (remember SVO or subject-verb-object). Unlike adjectives, which always sit on the left of the noun, adverbs can sit on either side†.
What’s important about the adverb is that it does modify or add flavor to the verb. Yet, it’s also a part of the written language that many believe is overused. Mark Twain is famous for saying:
I am dead to adverbs; they cannot excite me. To misplace an adverb is a thing which I am able to do with frozen indifference; it can never give me a pang. … There are subtleties which I cannot master at all,–the confuse me, they mean absolutely nothing to me,–and this adverb plague is one of them. … Yes, there are things which we cannot learn, and there is no use in fretting about it. I cannot learn adverbs; and what is more I won’t. —Mark Twain
In reality, the use of adverbs should be to increase awareness and understanding of the verb. As such, the use should become subtle instead of heavy-handed, adverbs should be used to augment when needed and not as a means of adding words to pad an assignment or book.
* — one of the cool things about the -ly suffix is that it can be added to anything to make it into an adverb. As a result, nearly anything can become an adverb, though not just anything is a proper adverb.
† - like much of the English language, use of adverbs is going to have an auditory or ‘feels right’ sense to it, meaning that some words will work better in front of the verb while others work better behind the verb.