The main building blocks of the English language are the different parts of speech. There are eight such parts, one of which is the adverb.
As the name suggests the adverb is a word that modifies a verb. Obviously, along with the noun the verb is a vital part of speech and the adverb gives an additional dimension to the action that is being portrayed by the verb.
Take a look at the following two sentences:
"The son LOOKS at his father."
"The son LOOKS GENTLY at his father."
Here the word GENTLY adds more meaning to the way a son looks at his father. In this example, GENTLY is the adverb modifying the verb LOOKS.