A metal sample is vaporised by a flame and is exposed to light from a cathode lamp that is made of the same metal as the sample being analysed. When the sample is vaporised, it absorbs certain wavelengths of light. So the light passes through the sample, some wavelengths are absorbed and the resultant light is focussed by a monochromator. The light strikes a sensor that measures the intensity. The amount of light absorbed (absorbance) is compared to known values on an absorbance curve (absorbance vs concentration (x axis)). Hence, concentration is obtained.
To use AAS, the type of metal present must be known because the cathode lamp must be made of the same metal.
HSC exam questions can ask you to describe the process, possible uses (heavy metal pollution detection, mining purity monitoring).
Some questions will also require you to analyse a function of absorbance and obtain concentration.