Proprietary Plugin for HPLIP
Some devices cannot be supported by HP's free-(open-source) software driver suite HPLIP (HP Linux Imaging and Printing) but need driver extensions or firmware files which are proprietary software (binary-only, no source code published) which cannot be distributed in this Printer Application or in Linux distributions. Therefore all these pieces of proprietary software are packaged together by HP to form a single plugin package which can be installed here.
In most cases, for ~95% of the supported printers you do not need to install it. Only if the printer model you have selected on the "Add Printer" page is marked with "requires proprietary plugin" or you find this remark in the print queue entry once you have set up your printer, you need to install this plugin.
With each new release of HPLIP by HP also a new version of the plugin is issued. You do not need to do any manual updates, this Snap gets auto-updated and if the update contains a new version of HPLIP, also the plugin (if it is installed) gets updated.
Plugin installation status
Plugin NOT installed