Symbols - What does heaven look like
The sign is connected with the spiritual path, and is also an indication that either everything is moving well towards your final goal or that you have achieved it. At one time the finger used in pressing the thumb would have been important.
The five fingers are then used to represent where the energy has got to in terms of the level and layers and the chakras - complex this I know, but there is a sort of correspondence working here between spiritual ascent and the ascent of energy up the spine.
The index finger thus represents the top most level and layer.
Thus the pressing of index finger and thumb, shows that you have broken through to the centre and are an enlightened being - success!
The sign is of course much misused these days although it is still used by those in power to indicate masonic knowledge, though not necessarily masonic achievement!