Potassium, along with sodium and chloride comprise electrolytes--compounds that conduct electricity when dissolved in water.

Potassium is important in regulating the activity of muscles and nerves. The frequency and degree to which our muscles contract, and the degree to which our nerves become excitable, both depend heavily on the presence of potassium in the right amount.

Diets high in potassium can help reduce the risk of high blood pressure. The 2004 guidelines of the Institute of Medicine specify a DRI of 4,000mg of potassium, though most Americans consume only half that amount per day.

Foods high in potassium include orange juice, potatoes, bananas, avocados, tomatoes, broccoli, soybeans and apricots.  

References

Debska G, Kicinska A, Skalska J, Szewczyk A. Intracellular potassium and chloride channels: an update. Acta Biochim Pol 2001;48(1):137-44. PMID:15860.