Explanation
A validation rule is a feature that allows administrators to define criteria for data entry or import operations and display an error message when those criteria are not met. For example, a validation rule can require users to select a product type or check a needs review box when saving an opportunity by using an OR function that evaluates both fields. If neither field is populated, then the validation rule will prevent users from saving records with an error message. References:
https://help.salesforce.com/s/articleView?id=sf.fields_about_validation_rules.htm&type=5