Spectrophotometry is a technique used to measure the concentration of a substance in a solution by analyzing its absorption or transmission of light. It involves passing light of a specific wavelength through the sample and measuring the intensity of light that is absorbed or transmitted, which is then correlated to the concentration of the substance through Beer-Lambert's Law. Spectrophotometry is widely applied in fields such as chemistry, biochemistry, environmental analysis, and pharmaceuticals for quantitative analysis, enzyme kinetics, and determining the concentration of various compounds.