Is your query specifically about gears on the Parva, or derailleur gears in general. I rebuilt a Parva for a friend last week and it had Shimano Tourney gears and Shimano shifters.
As
@DCLane said the left shifter moves the chain up and down the three chainring on the cranks, while the right shifter moves the chain up and down the cogs on the rear wheel. The big lever on each shifter uses pressure via the thumb to shift the chain from smaller rings to large, and the smaller lever behind it takes much less pressure to move the chain from big to small rings, basically using the springs in the gears to provide the movement.
At the front the bigger rings make the pedal cranks harder to turn and vice versa. At the rear it is the opposite with the bigger the ring the easier to turn.
Big ring at the front and small at the rear is used on the flat if you want to go fast, small ring at the front and big at the rear is used for steeper hills. Many different combinations in between for different terrain.
Trying to write it down makes it seem complicated, even to me and i wrote it, but once you ride with those gears for a while it becomes intuitive.
All this assumes, of course, that the gears on your bike are working properly.