When you rub two materials together, you observe a change in their electrical charge.
For a time these charges remain on the surface of the substance concerned (from a few seconds to several months depending on the material and the environmental conditions). They form what is called static electricity, generally present in small quantities. The more a substance is insulating, the greater the charge it accumulates.
Measurement is then essential to quantify these charges and to determine the points where they are generated.