Technically, a project can upload all kinds of files there. But the main point is use for development purposes. Developers share the resources and program code there and use it to collaborate. It's often not aimed at the end-user.

If you're lucky, you'll find a link to the project website either in the README further down, or in the description at the right, close to the top. That website is usually meant for the general public. Some projects also release binaries / executable files. It's also on the right in the "About" description or if you click on "Tags" and then "Releases".

Other than that, a lot of open source projects don't provide executable files. You'll have to install it with the package manager of your Linux distribution, or follow the instructions to build it yourself.