Indefinite pronouns are the words you use when you don't want to (or can't) name a specific person or thing — jemand ("someone"), niemand ("nobody"), etwas ("something"), nichts ("nothing"), irgendwo ("somewhere"). They're tiny, but they show up in almost every conversation.
A useful pattern: irgend- adds "any" / "some" flavour to questions and uncertainty (irgendjemand, irgendwo, irgendetwas), while niemand / nichts / nirgendwo are the negative pair. Jemand takes case endings like a normal noun (jemanden in the accusative, jemandem in the dative).
Exercise: Fill in the Right Indefinite Pronoun
The hint shows the English equivalent. Type the German indefinite pronoun that fits — and watch out for accusative forms like niemanden when the pronoun is a direct object.