On a subtractive synthesizer, envelope modulation is the amount that a modulation envelope effects a certain parameter (filter cutoff , oscillator pitch etc.)

Most synthesizers include an ADSR envelope, meaning:

• Attack
• Decay
• Sustain (sometimes a sustain level also)
• Release

Here is an example volume envelope from a software synthesizer called the TS404 :

Let's say a note is played. The attack value determines how long it will take for the sound to reach its peak after the key has been struck. The decay value determines the length of time it takes for the sound to decrease to the sustain level . The sustain parameter determines how long the sound stays at the sustain level . Finally, the release parameter determines how long the sound will keep going before fading out.

Envelopes can control many parameters besides volume, such as a filter, for example.