This is the final and one of the most critical layer of the Agile Planning Onion. Agile product development teams are constantly driving towards a state of continuous, adaptive planning, collaboration, design, development, testing and integration. This commitment fosters a dynamic, highly productive environment in which automation is critical and the output is always high-quality, valuable working software. You can read more about continuous integration In Matin Fowler's article.
You can review the Agile Planning Onion by clicking on the following diagram. I recommend to watch this video to get a better understanding of linkage between all layers.