When you choose this action for a validation rule, the value in a field is changed.
Specify a name for the action. This is just a label to help you remember what the action does.
Specify which field is to have its value changed.
Type the new value. Either type an expression yourself, or click Expression, which opens a dialog that can help you write the expression. The result of the expression must be a string or a numerical value. (If you want to write a simple text message, enclose it in quotation marks.)
Click OK.