Question

what is the role of the split ring in an electric motor

Answer 1

split rings are used to change the polarity of current after every half rotation so as to make the motor move continuously in same direction.

Answer 2

As we know, the electric motor converts electrical energy to mechanical energy and therefore will have moving parts. These moving parts (predominantly the armature of the motor) would still require a constant supply of electricity while moving. The split-rings facilitate this by acting as circular contacts that the carbon brushes can slide over while the armature is in motion, breaking the circuit at the split/gap halfway through the rings to accomodate the reversal in polarity.

I hope this is the answer you are looking for and (by God) I hope I didn't make any mistakes in that explanation.
In any case; Good luck, friend! :)