Every synth has at least two envelopes: an amp envelope (volume over time) and a filter envelope (cutoff over time). Attack is how fast it reaches full level, Decay how fast it falls to the Sustain level it holds while you press the key, and Release how long it fades after you let go.
Fast attack + short decay + low sustain gives you a plucky, percussive stab. Slow attack + long release gives you an evolving pad. The same synth becomes a bass, a lead or a pad purely through its envelopes.