Fluorescence Recovery After Photobleaching (FRAP) is a method of determining the kinetics of diffusion in living cells (usually) using fluorescence microscopy. The general method is to label a specific cell component with a fluorescent molecule, image that cell, photobleach a small portion of the cell, then image the recovery of fluorescence over time.

Why is there recovery of fluorescence? Diffusion or active movement of molecules within the cell replace bleached fluorophore with unbleached molecules that were located in a different part of the cell. Over time fluorescence in the bleached region recovers.

Above are example curves of FRAP of the fluorescent dye Oregon Green. In this experiment the researchers conjugated Oregon Green to the cell surface receptor "Toll-Like Receptor-4". They watched the recovery of fluorescence after different treatments. Fluorescence recovery indicated that the TLR-4 molecule was movable within the plasma membrane.
FRAP of GFP-E-APC in the larval optical stalk
from this page.
Requirements for FRAP:

What does FRAP tell us:

Here is a FRAP experiment using GFP-bacteria. The fluorescence recovery curve in the ROI is shown on the bottom.