Residual energy can best be compared to a kind of echo of an event. An example is a fire that has gone out. There is still residual energy in the form of heat and glow.
In the paranormal world, it is theorized that every thing leaves a kind of energy or echo.
Since it is a sort of echo, the energy represents an event that has taken place in the past. One cannot communicate actively with this event, and an apparition would just pass by.
Today EVP-sessions are widely used to determine whether a paranormal event is a residual energy or intelligent. By asking a question, and wait for the answer, investigators can determine intelligence. If there is no answer, or the answer is completely out of the context, one has found residual energy.