FD means Fluorescence Depolarization. Polarized light is light that has a specific oscillation direction. You can make polarized light by sending normal light through a polarizer. The rate at which such light is absorbed depends on the orientation of the molecules. Absorption of light brings a molecule in an "excited
state" in which it has a bit of extra energy. There are two extremes to consider: the cold limit and the hot limit. In the cold limit the molecules do not rotate much during the time they are in the excited state. In the hot limit the molecules rotate a lot while excited. As a result, the fluorescence emitted when the molecule
leaves the excited state is polarized strongly in the cold limit but much less so in the hot limit. It is clear then that, in general, the degree of polarization of the fluorescent light gives information about the rotational rates. However, the range of rotations (orientational order) also has a very strong effect on the FD. I have papers on FD in membranes and in muscle (see publications).