It's typical when you want to use advanced filter or any of the D functions
where you normally use a header and this one looks to see if the date is in
past send or equal to the date in B3, so if for instance you have a list and
you want to sum all entries in one column where another columns with dates
and only sum those rows that have a date that is less than or equal to a
certain date like yesterday or today if the date in B3 is today but if the
date are tomorrow it won't be included