For two lines #\blue k# and #\green l# with slope #a_{\blue k}# and #a_{\green l}# we have:

\[\begin{array}{c} a_{\blue k} \cdot a_{\green l}=-1 \\ \text{ if and only if }\\ \text{line }\blue k \text{ and line } \green l \text{ are perpendicular}\end{array}\]

This means that whenever #a_{\blue k} \cdot a_{\green l}=-1# the lines are perpendicular.

And also that if the lines #\blue k# and #\green l# are perpendicular then #a_{\blue k} \cdot a_{\green l}=-1#.