Mosquito dunks are a product designed to prevent mosquito larvae from growing and populating small bodies of water, such as fountains, animal troughs, and fish ponds. 
It contains a type of bacteria called Bacillus thuringiensis subspecies israelensis, which is beneficial as it infects and kills the larvae of flying insects, including mosquitoes, fungus gnats, and fruit flies. 
To use mosquito dunks you will need to fill a gallon jug or large watering can with clean water and allow the mosquito dunk to soak in the water overnight.  
Remove the remaining dunk which can be reused next time and use that water for watering your houseplants.