A noun may have many attributes.
Boy is a noun.
That boy may be tall or short, intelligent or fool, educated or uneducated, rich or poor.
What is the quality of that boy?
In order to express the quality of that boy, we have to use an adjective.
"That boy is a rich boy".
In this sentence, the word “rich” indicates that the boy is a rich boy.
That means that the boy has a lot of money.
Those words which tell us about the quality of the nouns (which might have been used either as the subject or the object) are called adjectives.
America is a country.
America is a rich country. (“rich” is an adjective)
Nile is a river.
Nile is a long river.(“long” is an adjective)
Harvard is the name of a university.
Harvard is the name of the oldest university. (“Oldest” is an adjective)
She is a student.
She is a beautiful student. (“Beautiful” is an adjective)
Water is an element.
Water is one of the essential elements. (“essential” is an adjective)
Other sentences written with adjectives:
London is a large city. Jacob is an honest man. We have had enough exercises. There has not been sufficient rain this year. The hand has five fingers. Most boys like cricket. I met a beautiful lady. The clever boy is rewarded. The beautiful painting is bought. The Indian goods have great demand in USA. The red car is preferred by all. The tall professor is on leave today. The idle servant was dismissed. He is an honest man. He is a mere child. It is sheer nonsense. He reads good books. This park is clean. Vishal became rich. He seems hungry.