First, this formula determines the first day of the next month *after* a given date. It does this my using EOMONTH to get the last day of the month, then adding one day:
Next, the formula calculates the number of days required to "roll back" to the last requested weekday in the month prior (i.e. the month of the original date):
Inside WEEKDAY, EOMONTH is again used to get the first day of the next month. From this date, the value for day of week is subtracted, and the result is fed into WEEKDAY, which returns the number of days to roll back.
Last, the roll back days are subtracted from the first day of the next month, which yields the final result.
In the general form of the formula at the top of the page, day of week is abbreviated "dow". This is a number between 1 (Sunday) and 7 (Saturday) which can be changed to get a different day of week. For example, to get the last Thursday of a month, set dow to 5.
Note: I ran into this formula in an answer on the MrExcel forum by Barry Houdini.