Dehydration is when the body is unable to get a replacement for an amount of water or salt lost from the body. The body functions with water and has a way of communicating when it is insufficient. When dehydration starts out, the symptoms are usually not major such as

Headaches

Dry mouth and dry tongue

Dizziness

Warm and dry skin

Passing of yellow urine or inability to pass urine.

When it is severe, it leads to very serious conditions such as

Heat injury

Urinary and kidney problems

Hypovolemic shock

Low blood pressure

Heart failure

Fainting and in extreme cases

Death