The envelope is a form of automation (automatic movement) built into synthesizers to control the value of a parameter (usually volume or filter cutoff frequency) over time. Envelopes are critical for giving synthesizers a more natural and interesting sound, as they can be used to simulate the volume envelopes of a real instrument. However, envelopes are also great for creating special effects. The most common type of envelope is called ADSR (Attack, Decay, Sustain, Release), referring to the different functions of each part of the envelope. You will see in FL Studio some envelopes are enhanced with additional elements, to suit some particular requirements for the specific instrument, however the principle remains the same.
ADSR Envelope elements: