Typically, there is one brush connected to the negative terminal and one brush connected to the positive terminal of the DC power supply.
There is some type of spring mechanism to provide the proper amount of tension to the brush so that proper contact is made to the commutator. If there is too little tension, the brush will bounce and arc; too much pressure, and the brush life will be shortened.
Motor Brushes are made of a carbon-composite material. Usually, copper is added to increase conductivity. Other materials may be added for longer life.
A common rule is that a brush should be replaced if it has cracks, nicks, any sign of wire wearing through, or if it is 1/2" in length or less.