This is a standard VLOOKUP exact match formula with one exception: the column index is supplied by the MATCH function.
Note that the lookup array given to MATCH (B2:E2) representing column headers deliberately includes the empty cell B2. This is done so that the number returned by MATCH is in sync with the table used by VLOOKUP. In other words, you need to give MATCH a range that spans the same number of columns VLOOKUP is using in the table. In the example (for Feb) MATCH returns 3, so after MATCH runs, the VLOOKUP formula looks like this:
Which returns sales for Colby (row 4) in Feb (column 3), which is $6,786.