The [Archive] button creates a ZIP archive of all the files contained in the site directory.

This is essentially a snapshot of the site at the time the archive was created.  The archive is placed in a subdirectory of the site directory and named using a time-stamp.  The archive can be moved to another location and/or the content restored at some later date.

Do not use the archive facility indiscriminately.  It can consume significiant disk-space.

The [Tidy up!] button removes all older versions of files.  This leaves just a single version of each the current files, preventing any sort of site content regression.  Version numbers on editable files are returned to ;1.  Tidying reduces the disk-space consumed.  It is generally a good policy to tidy after completing and publishing site modifications.