Some of the smaller devices running a sword front-end might not not have a network enabled module manager or network use might be slow and expensive. Users of these devices need to download their modules as zipped files and unzip them in the right place - e.g. a SD card.
It is also helpful for people who have no access to direct internet connections or live in countries with monitored access to the internet to be able to download (and possibly distribute) zipped modules. This is a zipped archive of a module in the file and directory layout used in the local module directory.
E.g. to create from your own module directory a zipped module of a uncompressed GenBook do this:
cd ~ftp/pub/sword zip zip/mybook.zip mods.d/mybook.conf modules/genbook/rawgenbooks/mybook