They are the voltage selectors. By using the two switches in combination you can select different power settings, Starting with Min1 being the lowest voltage, followed by Min2, then Max1 and finally Max2 being the highest. Basically, the thicker the metal you are trying to weld, the higher the voltage setting you need (and also the higher the wire feed speed). With a max rating of 100 amps the maximum thickness (i.e. setting at Max2) you are probably going be able to weld would be around 3mm I think. There is a calculator in the tutorial section of this site.