Diarrhea is a condition in which a person passes loose and watery stools frequently (more than three times a day). 
It is a condition which could be because of a number of reasons.
It can be due to viral, bacterial or parasitic infections or it can also be because of intestinal disorders, medication etc.,
In diarrhea, there is reduced fluid absorption from the intestine and increased fluid loss through the stools
Since, body is losing lot of fluid and also electrolytes (sodium and potassium) ions with it, re-hydrating (restoring/replacing the lost water) and electrolytes is important.
If the water and electrolytes are not maintained in their normal levels it will affect the proper functioning of the body.
So, Oral Rehydration Salt (ORS) solution containing all the components that are usually lost through the stool are given as treatment for Diarrhea.