An adverb can be a word (easily) or a phrase (last night) that describes or modifies a verb, an adjective or another adverb, and sometimes a sentence, but not a noun or a pronoun. We can identify a lot of adverbs by their endings. They end in -ly but not all, as some words that end in -ly are adjectives.
We crawled slowly around on our hands and knees looking for the needle. (Adverb slowly modifies verb crawl.)
I dreamed about you last night. (Adverb last night modifies verb dreamed.)
The monster was incredibly ugly. (Adverb incredibly modifies adjective ugly.)
Fortunately, we were in time to buy the last tickets. (Adverb fortunately modifies a sentence we were in time to buy the last tickets.)
Unlike adjectives, adverbs do not modify nouns
Incorrect: That woman has a beautifully daughter. (Adverb)
Correct: That woman has a beautiful daughter. (Adjective)
Incorrect: He found the exam quite hardly. (Adverb)
Correct: He found the exam quite hard. (Adjective)
Incorrect: We heard a loudly explosion and then saw thickly smoke. (Adverb)
Correct: We heard a loud explosion and then we saw thick smoke. (Adjective)
The adverb is an intensifier
Besides being a modifier, the adverb performs another function. As an intensifier, it makes the adjective that they modify stronger by giving it emphasis.
The weather is exceptionally cold at this time of the year.
He is downright rude to his parents.
The treatment is extremely dangerous for an old person like her mother.
The witch appeared hideously ugly in his dream.
The substance was found to be a highly addictive drug.
The adverb does not come in just one word. It can be a group of two or more words acting as an adverb phrase (or adverbial phrase). The adverb phrase does the work of an adverb in a sentence in modifying a verb, an adjective or another adverb. An adverb phrase may consist of one word or usually more words. The adverb phrases are in bold.