Fluorescence Correlation Spectroscopy (FCS) is a technique that records fluorescence signals from small observation volumes with high, typically single molecule, sensitivity. By statistically analyzing the fluorescence fluctuations of the small observation volume, one can obtain information on the frequency of occurrence and the characteristic time of any process that causes the fluorescence fluctuations. Typically (but not always), it is the diffusion of molecules in and out of the volume which causes the fluctuations. The FCS analysis in this case provides information about the average time the particles need to diffuse through the volume (i.e. the diffusion coefficient).
FCS provides information on the number of particles that are on average in the observation volume (i.e. the concentration).