What exactly is a coupling agent?
There aren't any, really. This is just biotech jargon -- the oxidative pathway, a series of enzymes that re-oxidises NADH (and other reduced molecules) with the ultimate goals of extracting energy and reducing oxygen to water (you know, the reason terrestrial animals breathe?) is said to "coupled" to a H+
dependent ATPase, to convert the potential energy into ATP, which cells use for later energy use. This "proton gradient" exists between two sides of a mitochondrial membrane, several of the proteins being embedded in this membrane. De-couplers cause this membrane to be "leaky", allowing this gradient to be lost without being used to make ATP. They're good ways to understand the process, or kill things, or help people lose weight ... sometimes all 3.