Sonic Pi is a fun free tool that uses the Ruby language, intended to teach programming to children or anyone. It is still in the final stages of development, thus isn't packaged or in the Raspbian repositories yet.
On the Raspberry Pi, to download it, open a terminal in the menus through Accessories -> Terminal. Go here and follow the steps: http://sonic-pi.net/get-v2.0. Tar is an archive file format, much like ZIP files.
tar -xvzf is one way to extract all files.
When it starts, type:
Then click run to see if it works. You probably won't hear anything, since you most likely do not have dependencies installed (Ruby, Supercollider and so on). One easy way to fix this is to run the following in a terminal:
sudo apt-get install sonic-pi
This will install Sonic Pi 1, which should pull in all needed dependencies for Sonic Pi 2. Note that running "sonic-pi" in a terminal will launch Sonic Pi 1, you still need to run Sonic Pi 2 in the correct directory as described at sonic-pi.net. Now restart Sonic Pi 2 and try "
play 80" again, it should work. If not, you may have issues with your sound output.