Dictionary Definition :
Physiology. Any of various ions, such as sodium, potassium, or chloride, required by cells to regulate the electric charge and flow of water molecules across the cell membrane.
Confused ? Don't be, it can be simple.

When we exercise we sweat. Depending on the climate, whether we are indoors or outdoors and on our own personal sweat rate we can loose quite a lot of liquid and salts.

Potassium, magnesium, calcium and sodium are all electrolytes lost through sweat.

Without replacing these electrolytes during and after exercise, we may experience muscular and overall physical fatigue, decreased endurance and muscles cramps.

So some energy drinks contain electrolytes to replenish those lost when you sweat.

To sum up - if you are exercising indoors, in a hot climate or general sweat a lot, then try using an electrolyte drink rather than your normal sports drink.