Friedrichs-Lee model is a model which couples the bare discrete states with the bare continuum states, where the exact solutions of the bound states, resonances and virtual states can be solved. We review how to use the exact solutions to understand some theoretical aspects of the resonances, virtual states and bound states. In particular, we show that the integration contours in the solutions make the norms for the resonances and virtual states not well-defined, new discrete states can be dynamically generated from the singularity of the form factors, and the behaviors for the near threshold states for the S-waves and higher partial waves can be different. We also review some applications of this model in discussing the properties the hadron spectrum.