Springtails (Folsomia candida) are tiny arthropods (1-4 mm) and are used as a "clean up crew" for your terrarium, eating mold and waste. They also make a great supplemental food for your poison dart frogs.
When adding springtails to your terrarium they will reproduce. Simply dump the entire springtail culture in your terrarium this includes the charcoal and mix into your substrate. Keep moist.