The repository has several puposes. All the users that are imported to Nprinting is stored in the repository. Also all the data about repart executions, distribution that you can see in the newstand is stored in the repository. I think also all the reports, connections, tasks and so on that you can se in the web console is stored in the repository.
So it's a database that stores a lot of the information about Nprinting. Both configuration, reports distributions to the newstand, task executions and so on.
NPrinting Repository abstract the persistence of data, the most important component is a database used internally by NPrinting for save: audit information, the data that you create in the web console and the references between these data. This include: Users, groups, recipients, published reports, connection cache, etc.
When you create a filter in NPrinting it looks for the possible data for the field so NPrinting uses the Repository service for save the data cached. Think in the repository as a service for manage the data persistence from NPrinting Server.