Multiplicity refers to the exponents of the factors. For example:
x³(x + 1)(x + 2)²(x² + 1) = 0
x: has a multiplicity of 3 because its exponent is 3
x + 1: has a multiplicity of 1 because its exponent is 1: (x + 1)¹
x + 2: has a multiplicity of 2 because its exponent is 2
x² + 1: has a multiplicity of 1 because its exponent is 1: (x² + 1)¹
Hope this helped!