Data validation is a feature in Excel used to restrict what a user can enter into a cell. For example, you can use data validation to make sure a value is a number between 1 and 6, make sure a date occurs in the next 30 days, or make sure a color is from an approved list only. Data validation can also be used to provide a dropdown menu with predefined choices when a cell is selected.
In the example shown above, data validation is used to provide a dropdown menu with four choices: Red, Blue, Green, and Purple. The validation is applied to the range B5:B16, and the dropdown menu appears when any cell in this range is selected: