The Clean function lets you remove certain files from the archive in an automated way. If you use a version control
system like CVS or Subversion, you will probably not want to include the hidden CVS and .svn folders in your archives. Similarly, BetterZip can filter out files that you extract from archives, e.g., the Thumbs.db files that Windows adds to folders to store thumbnails.
BetterZip uses search patterns (regular expressions) to find such files and removes them with the click of a button. To customize these search patterns use the Cleaning tab in the preferences window.
To learn what regular expressions are and how they work, read this Wikipedia article or search Google for a vast number of tutorials.