Period seems to be a date field. Period-14 is then 14 days earlier than the value of Period (i.e. two weeks earlier). Monthname returns the first day of the month of the date, in this case of Period-14. The second parameter 1 is an offset, so it doesn't return the month of Period-14, but a month later. Then this date is formatted using the Date function so it displays in YYYYMM format (e.g. 201204).
