If your site requires users to input data or information—such as in a survey, ecommerce checkout window, or email newsletter sign-up—you should provide labels and instructions so they know what is expected of them.
This isn’t just helpful for those who rely on assistive technologies or have issues with comprehension, it’s also a common sense practice for any user. Imagine finalizing an Amazon order only to find that none of the payment or delivery fields are identified. How are you supposed to know where to put your address and where to fill in your credit card number?
The point of this rule is to provide users with essential information regarding input fields. It is possible to include too much instruction. Superfluous information can make an interface as difficult to use as insufficient information can, so be sure to use your best judgement.