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.