You've already come across indefinite pronouns. They are a sub-group of pronouns and are used

– when it's not clear who or what is doing the action ("man", "jemand", "etwas", "jeder", "nichts", "niemand").
– when it's not clear how many are involved ("alle", "alles", "einige", "viele").

Examples:
Jemand muss den Maibaum aufstellen.
Es ist alles fertig für die Feier!

Indefinite pronouns can be used to indicate people or things. Some refer only to people and others refer only to objects.

Some of them can be combined with "irgend-" for enhancement: " irgendeiner", "irgendjemand", "irgendetwas". These pronouns emphasize the fact that it doesn't matter which one or who is meant.

Examples:
Irgendjemand muss den Maibaum aufstellen.
Vielleicht weiß der Reiseleiter irgendetwas über das Orakel.