ADSR stands for Attack, Decay, Sustain, Release. It describes how a sound changes over time:
Attack: how fast the sound goes from silence to full volume when you press a key. A piano has a fast attack. A violin bowing in has a slow one.
Decay: how fast it drops from that peak to the sustain level.
Sustain: the volume it holds at while the key is down. Not a time — a level.
Release: how fast the sound fades to silence when you let go.
Every sound you hear does this. A drum is all attack and decay, no sustain. An organ is all sustain. A pad synth has slow attack and long release. That's it. Now you know more about synthesis than 99% of people, whether you wanted to or not.