It is a third-party plug-in to Zotero that allows you to highlight quotes within PDF articles and add notes (annotate) PDFs saved to your Zotero Library and extract those to notes attached to your citations.
It also allows you to rename, organize, and synchronize attachments to items in your Zotero library. These organization features are especially useful for advanced users who plan on saving a large amount (over 200) PDF articles to their Zotero library. The free cloud storage offered by Zotero is usually enough for most users.
Here are a few things to consider before installing and setting up the connections between your Zotero, Zotfile, PDF reader/annotation software, your cloud storage system, your mobile devices, and your computer.
Consider what you currently use for cloud storage and research how that platform (Dropbox, OneDrive, etc.) would sync with your computer's files and with your mobile device. Consider if you'll continue to have access to that cloud storage once you graduate. For example, I am using OneDrive through the University of Redlands. If I change schools, will I still have access to these files? Probably not.
Research what PDF annotation software works for you (more info in the PDF Annotation tab in the left menu on this page). When I realized that the OneDrive app for Kindle Fire allowed me to annotate PDFs, I appreciated not having to install another app. I can also annotate on any computer that has Adobe PDF reader software with annotation capabilities. In short, considering the devices you're using and the apps/software available to you, will you be able to annotate easily and in all circumstances?