Minerals: sodium, calcium, magnesium & potassium,
When exercising you lose a lot of water through sweat, but you also lose minerals, including sodium. Sodium is very important in order to maintain the electrolyte balance in the body, and therefore it is important to replace lost sodium, especially when sweat losses are high and exercise lasts for more than 2h. In addition to just replacing the losses, adding sodium has several other benefits: sodium improves water absorption in the intestines, it stimulates thirst resulting in a drive for you to continue drinking, and sodium helps to better retain the ingested fluids. As you can image, all these things help you to stay hydrated!
In addition to sodium, also some calcium, magnesium, and potassium are lost through sweat (however in smaller quantities than the sodium). These electrolytes also have important functions in the body. For example, calcium is needed for muscle contraction, and also magnesium and potassium contribute to normal muscle function. This is of course very important during exercise, and therefore it’s a good idea to replace these as well!