It is charging the idol or image with the presence and power of the Deity turning it into a living image. It is just as the human body is a container that holds within itself various forces and energies and thoughts and feelings and the sense of self that give it its true value.
Now what nature does through aeons of evolution, man with the knowledge that Nature grants him can do it much faster. Thus if one has the proper occult knowledge even objects can be charged by a beneficial or harmful energies. That is how talismans work. Pranapratistha is a detailed knowledge of the same process. There is an elaborate process including selection of material that absorbs and keeps spiritual energy. There are rituals to invoke and establish the Presence of the Deity and then methods to keep the Presence there. This last part is most difficult. The Deity may come for a short while but often withdraws for several reasons. Then it is just a shell that sometimes gets occupied by other forms of energies. That is why most idols where the deity is invoked during a certain phase when they come closer to earth, such as Durga Pooja, the prana pratishtha is followed by the visarjan.
Affectionately,
Alok Da
