Neither "ADSR" nor "Envelope" are really such big words, nor are they very scary, but if you've never seen them used together they might seem intimidating. ADSR isn't a word, actually, it's just a short way of referring to Attack, Decay, Sustain, and Release, which again are not such big words, and other than "Attack" not very scary, but meaningless if you've never encountered them before.
So to demystify them here are some short definitions:
Attack is how quickly a sound reaches its full volume. Most instruments have a very short, or fast, attack. When you hit a drum , strum a guitar, or press a piano key, the sound is fairly instant (very fast attack).
Synths allow one to alter the attack to create new instruments and variations on physical instruments. For instance a Synth Pad might have long, or slow, attack where the sound gradually comes up from no volume to full.
Decay is how quickly the sound drops to its sustain level after its initial peak. For instance in GarageBand's Classic Rock Organ there is an initial key click which quickly decays to the sustained level of the key(s) pressed.
Sustain is the level of sound reached after the decay, the constant sound level that remains until the key is released.
Release is how long it takes for the note(s) to stop sounding after you stop playing it. Most instruments have a fairly short release time, but Synth Pads, in particular, may have a very long release time so the notes may keep playing for a couple of seconds after you've released the key.
For those that learn best through illustration you can think of the ADSR Envelope as looking like this:
(you can think of "Amplitude" as Volume)