Explain Like I'm Five

Electrolytes are chemicals that can ionize to carry current. Table salt is one electrolyte, but potassium and magnesium salts are also required by the human body. Nerves work by sending electrical signals, so they need these electrolytes in order for them to carry those signals throughout the body. The problem is, the amount of electrolytes available to nerves depends on the amount present in the bloodstream, and when sweat glands pull water out of the bloodstream, they also take some electrolytes with it. That's why you need to replenish them after sweating. If you don't, your nerves won't work as well and your muscles will have a hard time coordinating. The specific ions you need to organize muscle contraction are sodium, potassium, and magnesium, so if you're low in any of those, then you risk weakness and cramps. So it's not quite as simple as just drinking table salt.

(https://en.wikipedia.org/wiki/Electrolyte)