Certain calculations frequent contain repetitive operations and numbers. The K, constant, is a time-saving function which allows a single key stroke to perform a single operation and number on the displayed number.
For example, if 20 numbers are to be multiplied by -17.35, the K key can be used. Enter -17.35, then press the times key, then the K key; this "teaches" the calculator the needed operation. From this point on while entering a number and pressing the K key, the calculator will automatically multiply the displayed number through -17.35, saving you six key strokes.