Note that ARIA is an independent specification that's not specifically related to HTML5.
Client-side form validations help in giving immediate feedback to the user.
This is done by using validation attributes on form elements.
Validation attributes allow you to specify rules for a form input, such as whether a value must be filled in; the minimum and maximum length of the data; whether the data needs to be a number, an email address, or something else; and a pattern that the data must match.
This article leads you through basic concepts and examples about form validation.
For more information beyond this tutorial, see the Constraint validation guide. When you enter data, the web application checks it to see that the data is correct.
If the information is correct, the application allows the data to be submitted to the server and (usually) saved in a database; if the information isn't correct, it gives you an error message explaining what needs to be corrected.