(This is not intended as medical advice. It is a summary from my personal experience and research I've read.)

Various things can cause dehydration — overexertion, exposure, diarrhea, vomiting, blood loss. Drinking plain water for dehydration is a lousy idea; it can send you right into hyponatremia (low sodium, which in my personal experience is a huge big bunch of no fun) or hypokalemia (low potassium, which is reportedly worse).

The standard "everyone knows about it" fix for this is Gatorade — but Gatorade is full of way too much sugar. The somewhat more medical version is Pedialyte, recommended for keeping kids with diarrhea from dehydrating — but you might not have that on hand in the house.

A jury-rigged kitchen-counter substitute can be had as follows (pardon my barbaric American units):

1 quart clean water
1 to 2 tablespoons white sugar
½ teaspoon table salt
½ teaspoon baking soda (sodium bicarbonate)
¼ teaspoon salt substitute (potassium chloride)

Leave out the KCl if you don't have it (but it's worth keeping around). Don't leave out the bicarbonate.