Lumen is a micro-framework similar to that Slim and Silex to quickly start a small application or set up a web service without overload yourself with all the features of a full Framework. As Laravel, this framework was created by Taylor Otwell and according to benchmark on the Lumen site it’s would be one of the fastest, faster than Slim.
For my part I find this framework but it is entirely based on Laravel so for someone who was already familiar with Laravel, this framework will be just a simplification of Laravel. In addition, all projects with Lumen is compatible with Laravel.
If it turns out that in the future you need more features, you just have to copy your Lumen project and paste it into a Laravel project. No other configuration or adjustment is necessary.