Not trying to be stupid I know nothing about electrical what are the brush terminals

On motors/generators (starter, alternator, etc,) the brushes make the connection to the rotating armature. You can read up a little on motors and generators. Bottom line is, the brush terminals provide the access point for the outside world - to electrical leads. If you think of an electric drill for something simple...power is applied to the brush terminals, the brushes connect that power to the multiple rotor points as it spins. Slightly different system with DC power but the brushes serve same basic function - providing an electrical contact to something rotating. You may be dealing with a starter or alternator where faulty (worn) brushes or faulty brush terminal wiring is stopping power from getting to the starter, or from the alternator. Does that help at all?