Stability of closed-loop systems at -180 degree frequency
The Nyquist plot of a transfer function is just a plot of the imaginary part versus the real part of the transfer function. If you imagine a vector that starts at the origin and traces the curve starting at omega=0, then the angle of the vector is the phase shift and its length is the magnitude.

In the plot given as an example you can see that the gain at DC (the starting point of the curve) is close to 4 and the phase shift is zero. For higher frequencies the gain goes towards zero (because the curve terminates at the origin) and for a phase shift of -180 the distance from the origin to the curve is smaller than 1 because it is left to the -1 point and therefore the distance to the origin is smaller than 1.