SharePoint Form Validation provides a way to validate input content when you create an item in New-Form or edit an item in Edit Form. By default, SharePoint can do some simple validations such as "Required Field" on server side when you submit form. But you often need validations of various kinds on client side. For example, when you create a task, you would like the form to warn you when your input on "Start Date" column is greater than "End Date" column before the data can be submitted to server. SharePoint Form Validation can help you to achieve it.
SharePoint Form Validation can be used on almost all kinds of lists. For most columns in the list, you can add standard validation methods such as "Required", "Compare", "Regular Expression" and "Range" validators etc. After configuration, the form validation will take effect in the New Form or Edit Form page.