What is the purpose of feeler gauge?

Feeler gauges consist of thin slivers of metal which are graduated in thickness, probably from 0.003 ins up to 0.030 ins (ie 3 thou to 30 thou). This is assuming they are calibrated in inches, or they could be in mm. 3 thou would be about 0.075 mm and 30 thou would be 0.75 mm. You just find the appropriate thickness for the gap you have. Start with one thinner than the gap and work up until you find the one that just fits. Don't force the gap by trying to put in one that is too thick.