Rolls, uniquely in the modern era, use three spool engines.
N1 refers to the Low Pressure shaft (that’s the one you can see at the front and back of the engine)
N2 is the intermediate pressure shaft, and strangely N3 is the High pressure shaft.
They are commonly abbreviated to LP, IP, & HP.
Pratt & GE use two shaft engines i.e. N1 & N2.
I know this is over simplistic but it is outside my area of expertise what with me being a simple Avionic sort of chap, I have no idea why Rolls plough their own, particular, furrow.