Fleming's Reagent is a chemical solution used primarily to detect the presence of aldehydes and ketones. It is a mixture of iodine and potassium iodide in water, which reacts with these organic compounds to produce a color change, indicating their presence. The reagent works by forming a colored complex with the carbonyl group found in aldehydes and ketones. This property makes Fleming's Reagent a useful tool in organic chemistry for identifying and analyzing various compounds in laboratory settings.