An envelope is a special type of amplitude modifier that changes amplitude over time. The common purpose of an envelope is to give synthetic sounds natural shapes.
A natural sound has four stages: attack, decay, sustain, and release. During the attack phase, the sound ramps up from zero amplitude to its highest amplitude. During the decay stage, the sound ramps back down to a somewhat lower amplitude level. The sustain phase holds that amplitude until release. The release phase then returns the amplitude back down to zero.
We emulate this phenomenon electronically. Attack, decay, sustain, and release. We measure attack, decay, and release in time. We measure sustain in relative amplitude.
These don't need to be static properties. It can be interesting to change them over time. A slow attack on softer notes, for example.
Not all envelopes are ADSR envelopes. Some envelopes skip the decay and/or sustain phases. More complex models are also available.