Help , I can't understand.

Because that is what you get when you factor it:

 

Factor the following:
a^3-b^3

 

Factor the difference of two cubes. a^3-b^3=(a-b) (a^2+a b+b^2):
Answer: |  (a-b) (a^2+a b+b^2)

Why does  a³ - b³  factor into  (a - b)(a² + ab + b²) ?

[You orginally wrote  b³  instead of  b²  --  I'm going to assume that's a typo, because  b²  is correct,  b³  isn't.]

 

You can show that factoring is correct by multiplying the factors to get the original problem.

 (a - b)(a² + ab + b²)  =  a(a² + ab + b²) - b(a² + ab + b²)

                                  =  a³ + a²b + ab² - a²b - ab² - b³

                                  =  a³ - b³     (after simplifying the like terms)

 

Since it multiplies correctly, a³ - b³  factors into  (a - b)(a² + ab + b²)