Hydroponics literally means "working water" (hydro=water, ponics=work). Practically, it means growing plants without using soil, delivering nutrients only by a aerated nutrient solution.
Hydroponics systems can be either closed or open systems. In closed systems the same hydroponic nutrient solution is recirculated and the nutrient content is monitored and adjusted.
Keeping the nutrient balance in such systems is a challenge and the hydroponic nutrient solution has to be sampled and analyzed every week. The nutrient solution composition has to be adjusted according to the results.