What is an Adverb?
An adverb is a word that describes a verb, like the adjective is to the noun. If I quickly walked to the door, the ‘quickly’ is the adverb. You can usually tell an adverb by the -ly at the end of the word though that can be wrong in some cases, but overall they do end with -ly.
Why is using them so bad?
There really is no reason they are so bad aside from the fact that most authors overuse them and use them as a crutch to be lazy. Expand your vocabulary and find more powerful verbs.
What is the alternative? More powerful verbs. Remember that words hold power. They can deliver hard emotion and direct people down paths just from what is read or spoken. Expand your verb vocabulary and see what amazing new ways you can describe people and scenes.
I quickly ran.
Using ‘quickly’ seems redundant with ‘ran’. Running is pretty quick though there are the slower versions such as jog. Maybe I sprinted or rushed. I even could have darted toward the door. ‘Quickly ran’ is too juvenile. Mature it up a bit. Give the reader more to see.