An Envelope Generator is a circuit that generally produces a rising voltage followed by a falling voltage, adjustable over time.
It is most often used as control voltage for a VCA.
The various voltages involved are called stages, and there are various different envelopes with a different amount of stages.
The simplest envelope is an AD (Attack-Decay) or AR (Attack-Release) envelope with only two stages: One rising and one falling voltage.
Niklas Ronnberg AR envelope
A slightly more complex form is the ADSR that has four stages consisting of Attack, Decay, Sustain and Release.