Fig.: to approach or exceed known performance boundaries

MJ was a music and entertainment genius. By introducing his signature moonwalking, locking and popping, and street dancing, he pushed the envelope of the entertainment industry

MJ era un genio della musica e dell'intrattenimento. Con l'introduzione del suo caratteristico moonwalking, locking e popping, e street dancing, ha spinto oltre ogni limite lo show business, che da allora non è più stato lo stesso

The strikers chose to accept a good compromise because they understood that pushing the envelope would put them at significant risk

Gli scioperanti hanno deciso di accettare un buon compromesso perché sapevano che spingendo la protesta fino al limite avrebbero rischiato grosso

This expression comes out of the U.S. Air Force test pilot program of the late 1940s. The envelope refers to a plane's performance capabilities. The limits of the plane's ability to fly at speeds and altitudes and under certain stresses define what is known as its “performance envelope.”
This expression became popular, and entered the common language, thanks to the films The Right Stuff and Top Gun