2.21 explain that positive and negative electrostatic charged are produced on materials by the loss and gain of electrons

When two insulating materials are rubbed together, they become electrostatic ally charged. This is because the electrons are transferred from one material onto the other. For example, when polythene rod (this has a negative charge) is rubbed against a cloth, (this has a positive charge), all the electrons move from the cloth to the polythene rod. This leaves the polythene rod negatively charged (as it has gained electrons) and the cloth positively charges (as it has lost electrons).

A way of remembering this is with the anagram, NIGPIL (yes i made it up, if you remember it tough, it does work!). It stands for...
Negative Is Gain - as the material that gains electrons becomes negatively charged
Positive Is Loss - as the material that loses electrons becomes posotively charged

NOTE: Which material transfers which electrons will depend on the two materials involved - it will not always be the cloth losing electrons/the rod gaining electrons.