An adjective is a describing word.
Its role in a sentence or in speech is to describe a noun (a noun can be a name of a person, place, thing or idea), a pronoun (a pronoun is a word that takes the place of a noun such as I, you, he, she, it, we, they) and to quantify and identify them.
Yesterday I went to a big market in Bucharest. I bought some fresh tomatoes, some beautiful flowers and seven yellow bananas.
In the sentence above, the adjective big describes the noun market, the adjective fresh describes the noun tomatoes, beautiful describes the noun flowers and yellow describes the noun bananas.
Some and seven are also adjectives. You will find more information regarding this subject in Types of adjectives on our page.