FRET is the abbreviation of Fluorescence resonance energy transfer.
FRET describes the transfer of energy between two closely located chromophores. When the first chromophores gets excited, it will emit energy at a certain wavelength (different from the excitation wavelength).
This emitted wavelenth of the first chromophore will excite the second chromophore, when this is physically close to the first. It will then get excited and emit light at another (different) wavelength. This technique is used to detect interactions of proteins.