What is the function of split rings in electric motor and what is the function of brushes in electric generator?

Slip Rings in the axle of an electric motor are used to change the direction of flow of current through the coil in the motor, so that the torque on the axle is in the same direction throughout a rotation.

Carbon brushes are used  to  maintain a point of contact between the slip rings attached to the rotating axle and the power supply so that at each moment there is a power input to the coils.