Mostly sodium and potassium salts, these are needed for the body to function properly. This is why oral dehydration solutions contain these and sometimes more stuff so you will be able to recover. You lose both water and electrolytes quickly when you sweat a lot, vomit, or have diarrhea, and if you just drink regular water, you won't be able to absorb it properly.

Usually when someone is severely dehydrated, you give them fluids intravenously, but it isn't always possible, so there are rehydration solutions that can be used. If someone is dehydrated but not too severely, oral rehydration solutions are great.

This is also the reason why you should never drink distilled water, it lacks these elements in them, it will damage the cells in your body when it tries to reach equilibrium.