The page that Damkerng T. linked to basically gives the definition:
Perhaps the broadest category of creativity research is that of environmental "press," a term first coined by Murray to describe pressures (hence "press") that influence creative people or the creative process.
So it's a very specific jargon term, but it clearly means "pressures that influence creative people or processes". Pressures in this context means something like
1 b : the constraint of circumstance : the weight of social or economic imposition
or
5 : the stress or urgency of matters demanding attention