b^2-4 factorises into (b-2)(b+2), because b^2+2b-2b-4=b^2-4.
Another way to do this is to write b^2-4=0. So b^2=4 and b=√4. There are two square roots of 4: 2 and -2. These are the roots of the equation or zeroes of the expression b^2-4.
So the expression is equivalent to (b-2)(b+2), because putting b=2 or b=-2 will make one or other factor come to zero.