Periscope shows a tree of boces representing your USB storage's folder structure, where each box's color and size represents the disk usage of that file or directory. The contents of each folder are shown underneath the box for that folder, so you can drill down into the hierarchy.
Tap on a box to zoom to that box, or swipe and pinch to navigate around. The children of each folder are sorted biggest to smallest, left to right. Children that aren't big enough to show are grouped together into a grey box - zoom in or tap to see them.