Safer® Hydro FAQ: What Is A Nutrient Solution?
Plants grown indoors need to be fertilized, because they don't have access to the nutrients that are naturally found in soil. Liquid fertilizers make it easy to give indoor plants – hydroponic or soil-grown – a steady supply of the nutrients they need.
Most liquid plant foods come in concentrated formulas, so growers mix it with water to dilute it so plants don't get “burned” by excess nutrients. You use the solution of water and nutrients to feed and hydrate your plants at the same time.
When you make nutrient solution, try to make only as much as you need at a time – as it ages, the nutrients diminish and unhealthy fungi may begin growing in it.