The affinity between two molecules tells you how tightly they bind to each other. Affinity measurements are reported as the affinity constant, equilibrium dissociation constant, or Kd. The Kd and affinity are inversely related. The Kd value is related to the concentration of one of the binding partners and so the lower the Kd value — lower concentration expressed in molar values — the higher the affinity between the two molecules.
In a TRIC assay, the molecule you label with the fluorophore is called a target. The other binding partner — another protein, nucleic acid sequence, small molecule, or fragment — is called a ligand. To calculate the Kd, a constant amount of the fluorescently labeled target is mixed with a dilution series of a ligand. The recorded changes in fluorescence are plotted against the logarithmic ligand concentration to build a binding curve. The Kd is determined from the binding curve using the law of mass action.
The dose-response curves are obtained by plotting the Fnorm values (the ratio between the fluorescence values after and prior to the laser activation) against the ligand concentration. The data is fitted with a dose-response model that describes the law of mass action.