Everything is energy, the ground, the air, the light, and energy interacts with other energy trading characteristics or picking up new, totally foreign, traits. This interaction of energies is the focus of energy signatures. All living things radiate energy in the form of an aura and no-living objects displace universal energy enough to have a non-radiated aura around them. When auras come together they interact with one another and in so doing leave distinct patters and marks on each other. These patterns that are obviously foreign to the aura they exist on are resultant energy signatures from interaction with other energy. So, how is this useful? When a person is able to see auras or even feel them they can spot the areas where interaction has occured and if the new/foreign energy pattern is familiar they can tell who that person, or object has had contact with.
An energy signature will dissipate with time. The duration of the energy signature depends on the strength of the original energy. Signatures can be taken off an aura through aural cleansing, or they can be avoided altogether through shielding against such aural interaction.
Each energy signature is unique, like a fingerprint, and each signature for its duration holds a very thin thread back to its original source. This small connection with the creation energy can be traced (one that is able to do this is usually of master level) back to its origins to see who, or what each interaction has been.
Essentially energy signatures are energy left behind by common, or psychic everyday interactions.