Annual median $/SF is the median of each closing's own sale price divided by its square footage, over the twelve months ending on the date in the heading. Last year is the same measurement over the twelve months before that, so the two columns divide to give the change.
ARMLS closings is the number of sales the median is struck over, and is the denominator behind the figure beside it. A city is shown only where it has at least 20 of them in the period; hover a city to see what it had the year before.