Fluorescence Resonance Energy Transfer (FRET) is a technique used to study interactions between molecules at a very close range, typically 1 to 10 nanometers. It involves two fluorescent molecules: a donor and an acceptor. When the donor is excited by light, it can transfer energy to the acceptor if they are in close proximity, causing the acceptor to emit light at a different wavelength. FRET is widely used in biological research to investigate protein-protein interactions, conformational changes, and cellular processes. By measuring the efficiency of energy transfer, scientists can gain insights into molecular dynamics and the spatial arrangement of biomolecules in living cells.