VCA is short for “voltage-controlled amplifier,” and for all intents and purposes, you can think of it as an automated volume knob.
Level- In the original Juno synths, this acted as a master volume control, in order to balance volume levels between presets. Typically this gets set to the center zero setting, and volume can be added or subtracted by moving it up or down.
Env/Gate Switch- The original Juno synths included a single envelope generator that had to perform double-duty, applying to the lowpass filter as well as the VCA sections. In order to add some flexibility, the Env/Gate switch effectively disconnects the envelope generator from the VCA. If the Env/Gate switch is in the Env (up) position, the envelope generator affects amplitude (VCA) and filter cutoff (if the the VCF Env slider is up) simultaneously. If the switch is in the down Gate (down) position, the envelope generator still affects filter cutoff, but note amplitude will instantly turn on and off, like an organ.
LFO- Adds amplitude modulation from the low-frequency oscillator. Though the original Juno synths did not have an LFO mod slider, it's useful for tremolo and other volume-based effects.
VEL- Short for "velocity," this allows keyboard velocity to control the volume of sounds. Increasing the amount adds sensitivity with lightly played notes getting quieter.