can anyone explaine me how the fan works

when do I have to increase or decrease  the number and why

I don't have a clue


More fan > more details, less adhesion.

Usually you need more fan on small and/or detailed models.

On a coin size I would use 100% fan.

I can confirm what Venosa said and would add, that I would suggest to use less fan when printing large (flat) objects to decrease speed of shrinking whats the main reason for warping.