Ensure data added in your forms match your rules 

With validation rules you can ensure that certain restrictions are applied prior to form being submitted. For example, if date field is in the past display a unique error. 

To create validation rules, start by adding or opening any form in your pages

Next select Form Rules then select Validation Rules and press the Add New Rule button to create your first rule 

We'll add a rule like in our example - if Created Date is in the past show an error message. To do this we'll select the Created Date field and choose the is before today option from the drop-down. If this equals to true the error message will be displayed 

Users will see an error that looks like this:  

Here are some more examples on how and when to use validation rules: