A literary, somewhat archaic term for the general populace, often with a nuance of ordinary people as opposed to rulers or elites. Not used in casual conversation.
In old books, one can find the expression 'for the common people'.
A Sino-Japanese compound of 蒼 (blue/green, also used poetically for 'common') and 生 (life/people). The exact historical derivation is uncertain, but it is a classical Chinese term for the populace.