Continuous integration is the process of automating and streamlining the building, analyzing, testing, and archiving of your apps, in order to ensure that they are always in a releasable state. In a continuous integration workflow, you write apps locally in Xcode on your development Mac and check them into a source code repository. You then send them to Xcode Server for processing.
In Xcode on your development Mac, you set up bots that run on the server. These bots process your apps, using the source code in your repository, and report back the results. Each run of a bot is called an integration, and these runs occur regularly throughout the development life cycle of your app.
Xcode Server can be run on any Mac running Xcode, including your development Mac.