The fabricated spiritron world that was made inside Moon Cell.
It’s a “moon-powering, city-like engine” made to manage the inside of the massive Moon Cell.
In short, it is an additional engine and additional memory that Moon Cell added on while expanding itself.
From the perspective of a normal human, Moon Cell’s network is only seen as light, but a magus who can transform their soul into spiritron is able to perceive the information held within that light, and join in as a unit attached to the network. SE.RA.PH is also used to carry out the Holy Grail War, which is for the purpose of better knowing and understanding humans.
SE.RA.PH is scattered around within Moon Cell, and includes, among other things, parts which are already old, obsolete and abandoned, and also independent areas which still haven’t collapsed.

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA