A laser source of squeezed states of light is considered, where relaxation of the lower operating level is related to the cooperative spontaneous decay (superradiant laser). A phenomenological description of such a light source is proposed, which should give simple physical insight into the reasons why it provides such efficient squeezing. The concept that the active medium of an ideal superradiant laser can be completely described by a nonlinear susceptibility to the fields in the channels of generation and radiative decay is developed. An efficient squeezing occurs under the condition of saturation of this nonlinear susceptibility. In this case, the linear response of the medium to the sources of fluctuations in the generation and decay channels vanishes, which is related to leaving the radiation from the resonator.