They are little blocks of spring-loaded, carbon/graphite that remain stationary and deliver electrical power to the rotating part of the motor. Like a pencil they wear away.
YouTube has a video of how to replace - they are all much the same on all W/M motors.... here...
http://www.youtube.com/watch?v=0GmtRBiXLFc
Good luck...
Peccavi, June 2010