Sweat is mainly water but also has some salt in it. The main function of sweat is to control temperature, or lower. When the water in sweat evaporates, the skin is cooled.
You didn't include a story, but the hotter it is and the more you move, the more you will sweat. This will lower anyone's bodily water. It is best to stay hydrated, preferable with electrolytes since sodium is lost also when you sweat.
