For an electric motor or generator to function, the coils of the rotor must be connected to complete an electrical circuit. To accomplish this, a copper or brass commutator or 'slip rings' are affixed to the shaft, and springs press braided copper wire 'brushes' onto the rings which conduct the current. Such brushes provided poor commutation as they moved from one commutator segment to the next. The cure was the introduction of 'high resistance brushes' made from graphite (sometimes with added copper).
Although the resistance was of the order of tens of milliohms, they were high resistance enough to provide a gradual shift of current from one commutator segment to the next. The term brush remains in use. Since the brushes wear out, they can be replaced in products intended to allow maintenance. Metal fiber brushes are currently being developed again. These brushes may have advantages over current carbon brushes, but have not yet seen wide implementation.