We're writing an app, this is the public repository we're working on.
In a terminal, switch to the project directory (i.e. where you downloaded the repository to, or unpacked the zipfile) and run the following command:
meteor
After a while it will display "=> App running at: http://localhost:3000/" and you can visit http://localhost:3000 in your browser to see the app running.
(Don't close the terminal, or your server will be stopped. Only close the terminal after you're done and always remember to start it before starting to edit the files of the app.)
Now you can edit the files of the app, and improve it!
Have fun!