Not sure why you seem to be having difficulty understanding this (I thought my earlier response to your initial query was quite clear). It is simply the number of values (Open, Close, High, Low Prices etc.) used when calculating a function - in this case 'BullPower' but all the built-in functions are the same.

This number has the visual effect of 'smoothing' the line produced by the function - the bigger the number, the smoother the line. Just put a MA on any chart and play with the 'Period' variable. It should then be obvious as to its effect...