The first three lines simply get the current row values from the Month, Year, and Revenue YTD columns and store these values in the __Month, __Year, and __Revenue variables, respectively. In order to calculate the current month's revenue, we need to compare the current Revenue YTD value with last month's Revenue YTD value. We do this by using ALL to break the row context and get all of the rows in our table. Then, we use FILTER to filter down to the single row whose Year column equals our current row's Year column, __Year, as well as whose Month column equals our current row's Month column, __Month, minus one. We could use any aggregation on this single row, but for multiple rows, we would most likely want to use the SUM function.
Once we have the Revenue YTD value for the current row, __Revenue, as well as the Revenue YTD value for the previous month, __LastMonthRevenue, the revenue for the current month is simply the difference between these two values.