This sub-section of the guides goes into detail about services, also known as managers. Services are essentially a separate, child process, that is running in the background that is managed by a parent process (more specifically, the salt minion). If that process fails, salt will make sure that it is started up again and the error is reported as needed.
More importantly however, the default services that come with an AutoPi are the preferred point of communication between the Core software and their hardware counterpart. For example the OBD manager is responsible of communicating directly with the STN chip to get communications with a vehicle working. This makes services a very convenient place for operations that are closely related to specific parts of an AutoPi device.
It is also possible to create custom services that run on the device. You are able to define PIP package requirements for them as well. These services will be started alongside the rest of the services - when the device boots up.