What does it mean when it says 15x2 etc? the magnets have been wound twice 15 times? what does this do for the motors power?

I am assuming you are talking about a brushed motor.
15 single means 1 wire is wrapped around 15 times.
15 double means 2 smaller wires are wrapped around 15 times.
15 triple means 3 even smaller wires are wrapped around 15 times.

i think singles have more torque and low rpm power for high traction tracks while doubles or triples are smoothe for slippery tracks.

+1

a single wind is like having a 2-stroke dirtbike- narrow powerband
double-triple is like a 4-stroke dirtbike- broader powerband.

I like triple winds personally. Always made my truck good to drive.