Fundamentals are simple, it's how many ounces of silver you can swap for gold. Done properly, you can increase gold or silver holdings by swapping in and out when the GSR reaches favourable numbers. Unallocated swaps can be set up with Goldstackers.
GSR is high when PMs are low (favorable to hold silver) and GSR is low when PMs are high (favorable to hold gold). It is long term swings of at least a couple years.