A carbon brush is a sliding piece that transmits electrical current from the static part of a motor to the rotating part, minimising the sparking between the two contacts.
They are necessary for torque:
"For getting torque in the same direction throughout the rotation, it is necessary to reverse the direction of current flowing through the rotor winding in every half cycle. This is called “commutation” and it is achieved by supplying the rotor current through brushes. The brushes are fixed to stator and they connect to copper plates fixed to rotor. As the rotator rotates, the brushes supply current in the right direction." source
Downsides of a Carbon Brush:
They do have their downsides though, as they need maintenace because they're a wearable part. You may also have to periodically clean the commutator and the stator in the tool.