For a definition of Page Class Suffixes, see the Joomla! Glossary. You can use The Page Class Suffix parameter to create unique CSS classes that allow you to style one Menu Item differently than others.
If you enter a Page Class Suffix with a leading space, a new CSS class will be created. If the parameter does not have a leading space, the CSS classes associated with this Menu Item (for example, "componentheading") will be modified. The first method is normally preferred, since then you don't break any of the existing styling for the page elements, and you only need to add new CSS code for the new styling.
If you don't use a leading space, you will need to copy all of the styling code for the Menu Item classes and duplicate it for the new CSS class before making your CSS changes.
See the tutorial Using Class Suffixes in Joomla! 1.5 for a detailed example of using Page and Module Class Suffixes.