An adjective describes a noun or pronoun. It also gives a noun or a pronoun a more specific meaning.

It was about seven in the evening when the terrible earthquake of 1863 occurred.

It was a severe, lateral shock.

It had a peculiar, sickening, up-and-down movement.

In the sentences above, the words the, terrible, a, severe, lateral, peculiar, sickening and up-and-down are all adjectives - they modify, describe, or limit the nouns that follow them.