rake preview
is a command which launches your site running locally on port 4000. Therefore if you execute that command and browse to localhost:4000 you should see your site. The command will appear to be running constantly (until you ctrl-c out) as it is running a server.
This is covered under the blogging basics - http://octopress.org/docs/blogging/
rake watch
will also run continually as all it is designed to do is monitor for changes in your source and sass then if any are detected will automatically run a generate task.