The bicycle hub is one of the simplest bearings on a bike. A hub is the centre part of a bicycle wheel, the main components consists of an axle, bearings and a hub shell. Hub maintenance is essential for the smooth running of your hubs and freehub. hubs need attention if the bearings become loose, tight or gritty other wise you will loose performance and/or face the consequence of a catastrophic failure. The best way to fix is to take it apart, clean, add grease and replace the ball bearings if necessary.
For more information on bicycle hubs and common bicycle hub problems follow this link bicycle hubs