What is the usage of repositories and interfaces in Laravel?

Short Version

What is the usage of repositories and interfaces in Laravel?

Long Version

After developing projects using Codeigniter since 2 years, I stared to learn Laravel.

I downloaded a few projects lo learn how they are coded. Many of projects are I understand because they are used only models, views and controllers.

But one project has used repositories and interfaces. It is really hard to understand whats going on that project. So what is the usage of repositories and interfaces in Laravel? When should I use them?

via Chebli Mohamed