The voltage course of an ADSR envelope (see Figure below) is started if it is triggered by a gate signal, e.g. by pressing a key of the keyboard or by a sequencer. This gate signal (not shown in Figure) rises to a certain value, when the key starts (corresponds to time = 0 s in Figure) and drops to zero when the key is released (at begin of release in Figure).
If the attack (A) value is not 0, the voltage at the ADSR output is rising, followed by a decay (D). In the next phase the voltage is held constant at the value sustain (S) until the key is released. After the key is released the voltage goes down to zero, depending on the release (R) value.
How strong the envelope acts on a certain parameter (e.g. VCO: frequency or VCF: cutoff frequency or VCA: amplification, ...) is controlled by the depth parameter.
In concrete words, the depth is how high the controll voltage output is for the whole envelope.

An ADSR in action is shown in the following video, where a melody is played by a sequencer while the ADRS values are changed manually.