Worm castings is an organic fertilizer that comes directly out of the business end of a worm! It is basically black gold to your plants. 

Add it to your soil as an amendment when repotting, or sprinkle a few tablespoons onto the soil surface monthly. It will dissolve in water giving your plant a boost on watering day. 

For plants like cacti, succulents, snake plants, and ZZ plants, you can dissolve in warm water and then water your plants as normal.