Explanation
A transform map is a set of field maps that define the relationships between fields in an import set and fields in an existing table1. When importing data from a spreadsheet, creating a transform map is the step that defines where the incoming data columns will be written in the receiving table2. The transform map allows users to specify how to transform the source data into the target data, such as by using scripts, coalescing, or mapping assist1.
References
Transform Maps - Product Documentation: San Diego - ServiceNow1
Importing Data: Excel to ServiceNow - Finite Partners2