Simplified, their slopes multiply to -1.
What that means is that if there's a equation
[tex]y- y_{1} =m(x- x_{1} )[/tex] , if a line is perpendicular , that line's m value multiplied by this line's m value equals -1. For example, if m=1/4 for one value, the first thing we do is make it negative to get -1/4. Next, we switch the numerator and denominator to get -4. If there is a number like 5, the fraction of it is 5/1 (since 5 divided by 1 is 5) and the reciprocal (or the numerator and denominator switched) is 1/5. This also applied to y=mx+b.
Feel free to ask further questions!
