Question

What is meant by electrification by friction?

Answer 1

the frictional charging process results in a transfer of electrons  between the two objects that are rubbed together. Having an excess of electrons, the rubber balloon is charged negatively. Similarly, the shortage of electrons on the animal fur leaves it with a positive charge. 

Answer 2

This transfer of electrons takes place due to friction between the two objects. The object that transfers electrons loses negative charge (electrons) and the object that accepts electrons gains negative charge (electrons). ... This method of charging an object is called electrification by friction.