Some people get uncomfortable every time they see a letter of the alphabet mixed with numbers. They understand what to do with the numbers, but not those letters.
The letters represent numbers that change in value or are unknown. They are called variables since the numbers those letters represent vary from time to time.

A good example is pay. If a person is paid $20 per hour, to find his weekly pay, you would multiply $20 by the number of hours he works each week. The problem is that you may not know how many hours he will work each week. It may be 40 hours one week and 37 the next. However, if you represent the amount of hours with an h, you can say

The h is a variable that represents a real number that varies from week to week. The $20 represents the hourly rate of pay.
To work with any variable, you must know exactly what it represents. There are many variables in this book. As you work with them, you will learn what they represent.