The slideshow module, like the others, is self explanatory - it lets you build slideshows of selected images. Assuming you've selected your images using the library filters, such as ratings or picks, or selected them manually with your mouse, you can build a slideshow using those images. Alternatively you can select your images using Ctrl click in the filmstrip to select multiple items.
Like the previous modules, the left side panel has templates that determine the look of the slideshow, while the right allows you top modify and personalise your slideshow. Controls include the ability to change the colours, add your name as an overlay, set images as backgrounds and adjust drop shadows and so forth. Play around and find your ideal.
As an additional bonus, you can add a soundtrack to your slideshow by using the final right hand panel. This works with iTunes playlists and lets you select which playlist you want to use using a dropdown menu. A word to the wise - make a specific playlist beforehand to better control which songs you want to use. Finally the slideshow can be saved as a PDF, without music by clicking on the Export button.
It's worth mentioning that, on my computer at least, the slideshow is the weakest link in the Lightroom chain and continually crashes, so I don't use it too much. However I rarely make slideshows, so it's not a problem for me. I would be interested to hear from anybody else who either has problems, or has found it easy to use