This method of solving quadratic equations depends on a property of numbers called the zero product property.
The Zero Product Property
This property comes into effect when the product of two numbers results in zero.
The only way the product could be zero is if one or both of the factors is zero.
or 
Solving Quadratic Equations by Factoring
1) Write the quadratic equation so that all terms are on one side equation and zero is on the other side of the equation.
2) Factor the quadratic expression.
3) Apply the zero product property.
4) Solve the linear equations that remain after using the zero product property.
Examples:
Solving a Quadratic Equation by Factoring: Difference of Squares