From Joomla! Documentation
Module Class Suffix is a parameter in Joomla! modules. It is set Module: [Edit] screen under Advanced Parameters. Setting this parameter causes Joomla! to either add a new CSS class or modify the existing CSS class for the
div element for this specific module.
When Joomla! generates a module, it automatically create a CSS class called "moduletable" to allow styling of the module -- for example,
To create a new class, enter the parameter with a leading space. For example, entering a space plus "myNewClass" will create a new CSS class called "myNewClass". The HTML will be changed to
<div class="moduletable myNewClass">
To change the name of the existing class, enter in the parameter without a leading space. For example, entering "_mySuffix" (no leading space) will cause the HTML to changed to
Generally, it is recommended to use a leading space to create a new class. This way, CSS styling for this module that uses the standard class names will continue to work. You can use the new class name to add any desired styling to the module without needing to re-create all of the existing CSS code. Note that, if you create a new class name, make sure it has a unique name and doesn't conflict with any existing class names.
See Using Class Suffixes for more information.
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 for a detailed example of using Page and Module Class Suffixes.