There are many answers, as it is different things to different people. In terms of a schema, normative rules define the behavioral program of a people while language frames their common assumptions.
Culture is also a means of transmitting behavior and values across generations.
Further, it is a communication system, and it has been analyzed as a means for distributing both human and natural energy. Perhaps it is this as well. As a cultural constant, stupidity is routinely transmitted from one generation to the next by the time-honored mechanism of the vicious cycle. Poorly adjusted children mature into maladjusted adults, then using the same techniques their parents used on them to raise yet another generation of misinformed conformists or malcontented sociopaths. If there is some selection pressure acting to weed stupidity out of each generation, it is, apparently, easily offset by a willing disposition of people to spread it and encourage its continual, spontaneous synthesis.