Dear Shakeel Ahmad , usually an absorption edge is associated with the excitation of a bound electron into unoccupied states, or into the continuum, when the photon energy is increased. Since each bound electron has a characteristic binding energy, the energy required to excite this specific electron has a characteristic value as well. since a new absorption channel is opened when the photon energy increases to the value of the electron binding energy, a sharp and distinct increase of the absorption cross section (more specific the photoionization cross section) is observed - the absorption edge. Come back if you have more questions, Dirk

The absorption edge are is the region before the maximum value of absorption