Lookup Value — This is the rep number that you are trying to find. In the current example, it would be the value in C2.
Table Array — This is the range that contains the key field in the first column and the lookup values to be returned in the subsequent columns. You will specify this as an absolute reference, such as $H$2:$I$21. The dollar signs ensure that Excel always points to this range, even as the formula is copied down the worksheet.
Column Index Number — In the current example, a column number of one would return the rep number found in column H and a column number of two would return the rep name.
