The :-moz-placeholderpseudo-class represents any form element displaying placeholder text. This allows web developers and theme designers to customize the appearance of placeholder text, which is a light grey color by default. This may not work well if you've changed the background color of your form fields to be a similar color, for example, so you can use this pseudo-class to change the placeholder text color.
The HTML element <input> is used to create interactive controls for web-based forms in order to accept data from the user. How an <input> works varies considerably depending on the value of its type attribute.