<style type "text/css"> </style>
Are opening and closing parts of the styles that tell the browser that the style sheet starts "here and stops "there."
<!-- -->
These are the opening and closing parts of a traditinal HTM comment tag. This is used to explain that the "stuff" inside shouldn't be interpreted as HTML code.
h1
This tells us the style should be applied to the h1 tag.

background-color: #FFFFFF;
font-family: Arial, Helvetica, sans-serif;
font-size: 12px;
font-weight: bold;
color: #336633

All of this information is about the style, including the background color, font family, font size, font weight, and the color of the text.