To push the envelope is an idiomatic expression that comes from aviation terminology. An envelope is the set of limits for the performance capabilities of a machine. So, in this context, the expression means to increase the capability or a machine or to make it function past its limits.
It has also come to be applied to other situations. For example, when Calvin Klein first published very sexual photos for his underwear advertising campaign which were posted throughout New York on billboards and buses, you could say that his approach pushed the envelope beyond what was socially accepted in advertising. So simply put, to push the envelope has come to mean: to go beyond and stretch established limits, to pioneer.