You want to display some hint or help text within a form field.

The HTML5 placeholder attribute is used to specify instructional text when the user is not focused in the form field:

<form>
    <fieldset>
        <legend>Contact Information</legend>
        <p><label>E-mail address <input type="email" name="email"
            placeholder="user@domain.com"></label></p>
        <p><label>Web site <input type="url" name="website"
            placeholder="http://www.domain.com/"></label></p>
        <p><label>Telephone number <input type="tel" name="phone"
            placeholder="123-123-1234"></label></p>
    </fieldset>
    <p><button type="submit">Submit</button></p>
</form>