The GL Mapping sheet is a spreadsheet that tells Insight where to look within your imported data for the financial information it needs to place on the Data Entry (D) sheet.
When you customize your GL Mapping sheet, you are telling Insight which GL numbers in your data correspond to each Data Sheet item. Then, once Insight understands your general ledger structure, it can easily import the data you've extracted from your data processing software.
NEXT: Understanding The D Sheet Row Number and Category Columns