You don't need to understand what this tool is to know how to use it, but if you want an explanation here is a video.
First start by laying the trend at the market top, then count from there the next 3-4 lower lows, lay the trend, and extend the fibonacci tool part to the next closest lower high.
That's it! Now you just keep an eye out on those levels.
An important Note
These levels are to be used as areas to determine where to buy or sell and not specific price point. This is too help you find a better entry on average and potentially not miss the next run up.