What is this potentiometer for?
What is the potentiometer doing here? Why is it necessary?

It's there to control the &quot;offset voltage&quot; of the LCD, which is what controls the contrast of the display.

My lcd screen doesn't show anything. Does anyone knows why??

Maybe you wired it wrong. Maybe you didn't put the potentiometer in the circuit because you didn't understand what it is for. Maybe you just need to adjust the contrast by turning said potentiometer. It's hard for us to tell. By showing us what you intended to build and then telling us that what you actually built doesn't work, there is nothing we can tell you. There must be some difference between what you intended to build and what you did build, but we can't see what you built, so we can't have any clue.