Making advances in the art or science. For example, CGI is an advance in movie making and it's getting better with every cartoon movie. That's an advancement of the art that isn't just "art for art's sake", rather everyone benefits from it. In contrast, making a movie that only 5 people can understand may be "pushing the envelope", but it's only to the benefit of a handful of people.
Thanks. Sometimes - actually, much of the time - when “pushing the envelope” is used, it means pushing the bounds of what used to be called morality and decency even further into the abyss.
I have no clue where the term originated, I should look it up.