When your app is ready or at least when you want to install it on a device you need to Generate your app.
Generate will create a Build that you will be able to install on iOS, Android or run as HTML5.
Note that to install on iOS you need to have an iOS provisioning.
Unfortunately that's not the easiest thing to do.
To start you might want to Preview your app on screen or use a simulator.
To generate your app simply click on the 3 dots next to your app name and then click "Generate a build" in the dropdown
Choose the target you want to build for.
Choose if you to "Generate" Or "Generate Release Candidate", learn about the type of releases (Release Candidate, Commercial etc..)
"Generate" This option lets you see most incremental changes to the app without rebuilding it. It is only used during the development phase of your project.
"Generate Release Candidate" A release candidate build is very similar to a commercial build of the app. You can only create a commercial build once you have built and tested a release candidate.
Once you Generate an app you will see receive an email that your app is ready to install or view.
See also: Installing an App