Potassium is an inorganic chemical needed for the proper working of living cells. Potassium is especially important for the nervous system for the conduction of nerve signals. Potassium like sodium is a very reactive metal, so is only found as a chemical salt as potassium chloride, an impurity in sea salt. or potassium nitrate or saltpetre.
From a health perspective, it is best to keep the proportions of sodium and potassium in similar proportions, i.e. in balance to each other, but this can be thrown out of kilter by eating too much sodium chloride, salt. Fruits and vegetables contain decent amounts of potassium - bananas are especially good sources.