This is a piwik web analytics plugin. Piwik is a downloadable, open source (GPL licensed) real time web analytics software program.
It provides you with detailed reports on your website visitors: the search engines and keywords they used, the language they speak, your popular pages… and so much more.
To install it drop the piwik folder in the silex plugins folder.
You can activate it for a specific site or for the entire Silex server (for all sites hosted by your Silex server) through the Silex manager.