Allllrighty, I think I'll have a go at this one. G/A, I'm an aengineering student, so I'm sorry if there's a lot of technical language in my answer.
Well G/A, the "envelope" in this instance is not a white paper package to put letters into, but something known as the "flight envelope". If you draw a graph with an aircraft's speed on the x axis, and its altitude on the Y axis, then the "envelope" would be the area within the graph in front of you where the aircraft would be able to fly without stalling or crashing. If you're too slow, or too high, not enough air flows on the wings, and the aircraft either re-enters the flyable envelope or crashes. If an aircraft goes too fast, the air hitting it will start to weaken the airframe, and so something will break.
Back to the term "pushing the envelope". Each aircraft type has different size "envelopes" within which they operate. When fighter planes were/are being developed, and a prototype is built for flying tests, the test pilots deliberately fly an aircraft to the limits of the envelope, pushing the aircraft ever higher and faster.
To "push the envelope" is to push an aircraft, or anything else for that matter, to its very limits.
Does that answer your question?