For a definition of Module Class Suffix, see the Joomla! Glossary. You can use The Module Class Suffix parameter to create unique CSS classes that allow you to style one module differently than others.
If you enter a Module Class Suffix with a leading space, a new CSS class will be created. If your parameter does not have a leading space, the CSS class "moduletable" is modified. The first method is often preferred, since then you don't break any of the existing styling for the module, 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 "moduletable" class 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.