If a variable is dependent upon another variable, then its values will change as the value of the variable that its dependent upon changes.
Work Step by Step
For example take the function y=x+1. y is the dependent variable because as the values of x changes the values of y change also. The values of y derive from the value of x so therefore it is dependent upon the values of x. When x=1, y=2. When x=2, y=3. When x=3, y=4 and etc.