If you mean the charging light, it indicates either a bad earth or the brushes need changing, or both.
The light normally comes on when the engine is stopped, the battery voltage is fed through the field coils and to the bulb, thus giving a field boost when you start up.
If it comes on while running, that shows a fault in the field circuit of a back feed into the battery circuit, both of which need attention asap.
Check the brushes first, it's not a bad job, depends on your alternator.
Peter