When you share your project in configuration management system, you’re only sharing source code and project settings.
In these project settings are the packages dependencies (I mean the ones configured in Properties > Open AT Application page.
In order to be able to build this project on another system, you’ll need to install these package dependencies on this system, and that’s the purpose of this option.
As a repository, you can either use the official Sierra Wireless one (but it will install all packages from it, which may be too much…), or a folder containing the subset of packages you really need for this project (that you can copy from your desktop DS package path).