Basically, ‘prime rate' is the rate of interest that is decided by nations (U.S.A) largest banks for their preferred customers, having a good credit score. Much ‘variable' interest depends on the ‘prime rates'. For example, the ‘APR' (Annual Percentage Rate) on a credit card is 10% plus prime rate, and if the prime rate is 3%, the current ‘APR' on that credit card would be 13%.