The division (B2/$A$2) yields a decimal less than 1 (the $ signs keep the second reference on A2 instead of dragging it down thru the column). So 1 minus this decimal yields another decimal less than 1. For instance if the days absent are 7, the formula alone yields 0.3333333333. One minus this is 0.6666666666, and formatted as a percentage it's 66.666666...or 66.67 as you show. Basically if you entered the number of days attended, instead of the days absent, you could take out the "1-", because the percentage you want is the days they WERE there, the "1-" just inverts it from days missed to days present.
EDIT: late again...lol, oh well now you have 2 answers.