Screen Rotate is a system tool that lets you use your display in portrait mode. The developer of this app developed it to use his Macbook Air as an e-book reader, by holding it like you would hold a book. The Macbook Air, being light and small (at only 11 inches), can comfortably be used like this. Personally, I would never use a computer like that, because it is simply not the way it was intended to be. I guess if you don't want to or can't buy an iPad, which is a device that is best used in portrait mode for reading, Screen Rotate can be of use.
When you run the application, you will see a small floating window that has three buttons: W, L, and R. Those stand for Widescreen, Left and Right, respectively. By default, your Mac's screen is in widescreen mode. When you click on L, it turns into portrait mode, with the top of the screen on the left side of your display. When you click on R, the screen also goes into portrait mode, but the top is on the right side of the display.
Screen Rotate has an effect on the display and the trackpad, on Macs that have one. For example, on the Macbook Air, if you click on R, the display will be in portrait mode with the top on the right part of the display, and the trackpad will exhibit the same behavior.
In conclusion, Screen Rotate is an interesting application for users who want to turn their Mac's display in portrait mode. I would not recommend using a Macbook as an e-book reader, but there are other interesting uses for a display in portrait.