Filter floss is used to remove particles floating in your aquarium water.
These particles are generally decaying fish waste and fish food, but may also include particles of dead leaves.
Your filter floss also serves as a breeding ground for beneficial bacteria that use the contaminants in the water as food, and converts it to less toxic chemicals.
It is important to replace your filter floss approximately once a month.
NB: Never replace all your filter floss or you will remove most of the beneficial bacteria living in your filter.
I generally replace about 50% of the filter floss once a month.
This gives the bacteria time to re-colonize the new filter floss from the 50% left over.
The next month I'll replace the other 50% of floss.