Activated Charcoal removes chemical impurities from your aquarium water.
These impurities are in most cases invisible to the eye, and may include chemicals such as chlorine, chloramines and the like.
Activated Charcoal needs to be completely replaced once a month as the charcoal may become saturated with the filtered out impurities and will then start releasing these back into the tank with disastrous consequences to your aquarium fish.
Before you add your new charcoal to your filter rinse it under some tank water to remove any small charcoal particles from your charcoal.
Remove the cleaned charcoal and add it to your filter.
Throw away the tank water you used to clean the charcoal.