Surface Plasmon Resonance, SPR, is a physical phenomenon that occurs when (polarized) light hits a metal film at the interface of media with different refractive indices. Surface plasmons are collective oscillations of free electrons and these can be excited by focusing light onto a metal film through a glass prism. The subsequent reflection is then detected.
At a certain incident angle known as the resonance angle, the plasmons are set to resonate with light which results in absorption of light at that angle. By measuring the intensity of the reflected light beam at several incident angles, the resonance angle can be found by determining the lowest light intensity. This is sometimes referred to as the SPR dip, or dip angle.
When a molecular binding event takes place on or near the metal film, the dip angle shifts. By measuring the change of dip angle in time, binding events can be monitored and binding kinetics may be studied. The reverse process, (molecular) dissociation, can be studied in a similar way. See the animation below for a simplified representation.