Beamer is a Latex document
class that allows you to make pdf
files suitable for talks. It's what mathematicians use instead
It should be completely self-explanatory. Run Latex on it in the usual way and view the results as pdf.
My personal preference is for a minimal style: plain white background, no boxes, and no navigation symbols (those icons you often see at the bottom-right corner of people's slides). The sample file uses that style. If you don't like it, see the manual for how to change it.
Incidentally, the manual is quite well-written, despite its length. If you're going to do anything at all fancy with Beamer you'll probably want to use it.