This is the same as setting the Required field property to Yes.
However, when you enable the Required property and a user fails to enter a value, Access displays a somewhat unfriendly error message.
The steps in the section Add a validation rule to a control on a form, later in this article, explain how to add rules to the properties in individual controls.
Your validation rules can contain expressions — functions that return a single value.
Usually, a record validation rule compares the values of two or more fields.