The basic principle of displacement chromatography is: there are only a finite number of binding sites for solutes on the matrix (the stationary phase), and if a site is occupied by one molecule, it is unavailable to others. As in any chromatography, equilibrium is established between molecules of a given kind bound to the matrix and those of the same kind free in solution. Because the number of binding sites is finite, when the concentration of molecules free in solution is large relative to the dissociation constant for the sites, those sites will mostly be filled.  This results in a downward-curvature in the plot of bound vs free solute, in the simplest case giving a Langmuir isotherm. A molecule with a high affinity for the  matrix (the displacer) will compete more effectively for binding sites, leaving the mobile phase enriched in the lower-affinity solute.  Flow of mobile phase through the column preferentially carries off the lower-affinity solute and thus at high concentration the higher-affinity solute will eventually displace all molecules with lesser affinities.