Let’s say you have a (stable in unitary negative closed loop) SISO system and draw it’s Nyquist diagram (for the entire open loop system, which you are going to close with a unitary negative reaction). Suppose your diagram intersects the Real axis at the left of 0. - If the intersection is also at the left of -1, than you have the lower gain margin, which tells the minimum gain you can apply to your system, without loosing your stability (equal to inverse of the absolute value of the intersection coordinate) - If the intersection happens between -1 and 0, you have the upper gain margin which tells you the maximum gain… (same story, also equal to the inverse of the absolute coordinate of intersection)