Data validation checks that information follows the rules an app expects before it uses or saves it. It is like checking that a form has a real email address in the email box and not a phone number.
A sign-up form might require a name and check that a password is long enough. The server should also check information it receives, even if the page already checked it in the browser.
Validation helps catch missing or incorrectly shaped information, but it does not prove that the information is true. For example, a well-formed address could still be the wrong address.