The Blackphone file manager is based on the CyanogenMod file manager, which was originally developed by the CyanogenMod team and released as an open source application.
This app gives you access to your phone's file system and enables you to perform file management tasks such as copying and deleting files, much like Windows Explorer or the Mac Finder.
The double column of eight dots on the lower left side of the screen is an "actions" menu. The bottom right contains other useful icons to give you access to history, bookmarks, your SD card and search. Note File Manager places you in '/storage/emulated/0/' by default, use the icon that looks like an SD card to go above that level. Please note that if you to choose to use an SD card, this is not encrypted.