Potasium ions (K+) are vital minerals needed in out diet. Our body needs potassium for the nervous system to work effectively and for cellular osmotic pressure to be maintained. Potassium ions are generally found in cells (and sodium ions found outside cells). The sodium / potassium ATPase pump is responsible for maintaining the correct concentration of the ion. It is also involved in muscle contraction (although this is linked to the nervous system).
Deficiencies of potassium are very rare as it is found in all food sources. Bananas in particular are good sources of potassium.