What does the red light on my charger mean?

The red light on a charger can have several meanings:

Red light steady: Possible that battery is too hot (just taken off a tool that was used for a longer period) or cold (in car overnight and got really cold), give the battery time to return to normal temperature and try again
Red light Flashing: Battery faulty -> It will need to be replaced

On rare occasions, if the charger is faulty, the only way to eliminate this is to trial another battery to ensure the charger is in working order.