The button creates an archive of all the files contained in the site directory.
By default this archive is a BACKUP saveset.
Setting site configuration directive [archive-zip] makes it a ZIP archive.
This is essentially a snapshot of the site at the time the archive was created. The archive is placed in the subdirectory [._ARCHIVE] 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 significant disk-space.
The 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.