Each atom emits light of certain wavelengths when exicted, this acts like an atomic fingerprint since it is unique to an atom.
So a lamp is set up containing these excited atoms (ie they emit certain wavelengths), now the atoms also absorb the wavelengths they emit, so depending on how much of the light from the lamp is absorbed dertermines the concentration.
Standard Solutions are used to find out the absorbance so that a calibration curve can be drawn up and the concentration of the unknown found.