The document discusses data validation in Excel, which is used to ensure valid data entry. It describes how to select cells for validation, open the data validation dialog box, and specify validation criteria like limiting values to a certain data type, range, or list. Examples shown include validating for whole numbers between a minimum and maximum, text lengths less than a set number of characters, and selecting from a predefined list. The purpose of data validation is to restrict invalid or erroneous data from being entered into cells.