A phrase used to describe the averageness of a group of people's actions or behaviours.
Also meaning, nothing special, just average, not 'hot' or on fire.
'They' can be replaced by another pronoun if used to describe a singular person/place/event:
e.g. he/she/it didn't set the world on fire
John: Did you like the concert? Were the Rolling Pebbles any good?
Tim: Oh well look, they didn't set the world on fire but they were alright.