It means that if the product of two or more numbers is equal to 0, then one or more of them must be equal to 0.
To illustrate,
If a · b = 0 , then either a=0 or b=0, or both a=0=b.
To simply put, a product equal to 0 must have a factor/s of 0.
Also, this property is most commonly used when solving for roots of quadratic equations.
