Indefinite pronouns are words like one, none, all, some, few, everyone, everybody, anyone, anybody etc. These are often used to refer to people or things in a general way. Note that an indefinite pronoun does not refer to any particular person or thing.

Examples are given below.

One can’t complain.
One does not know how to behave in such a situation.
Everybody loves beautiful things.
What is everybody’s business is nobody’s business.
Nobody came.

Notes

In British English, the indefinite pronoun one, if used in a sentence, is used throughout. In American English, it is common to use he or she to refer back to one.

One must take care of one’s health. (GB)
One must take care of his / her health. (US)

Pronouns referring back to indefinite pronouns

When sex is not known we usually use a pronoun of the masculine gender to refer back to an indefinite pronoun.

Everybody must do his duty.

Note that many people consider this usage sexist and prefer using the expression he or she.

Everybody must do his or her duty.

Instead of he or she, some people prefer using the plural pronoun they.

Everybody is dissatisfied with their lot.
Each of them raised their hands.

Note that an indefinite pronoun can be followed by a noun. In this case it acts as an adjective.

Every child is beautiful. (Here the indefinite pronoun every acts as an adjective modifying the noun child.)
Few children like homework. (Here the indefinite pronoun few acts as an adjective modifying the noun children.)