Power Points can be uploaded to XpressU and used in presentations. In PowerPoint, each page is also called a Slide. We know it from slide shows, where pictures are shown one after the other. In the old days, we showed pictures as Slides through a projector.
So a Slide is a page in a PowerPoint.
XpressU consists of Frames that together form a Timeline.
A PowerPoint consisting of, e.g., 10 slides you can place each slide in a frame. Then XpressU can show one slide at a time by going from frame to frame.
In summary: Slide refers to pages in a PowerPoint and Frame refers to each individual part of the Timeline in XpressU.