The JobScheduler API let's you schedule a Job (or Task) inside the app framework. This system executes the job based on constraints, and the service encapsulates the code to be run inside the service class.
Basically when the service is created, the code is executed and then interacts with the UI through Broadcast Messages.
There are plenty good things about using this API. The JobScheduler API run in the main execution thread, but can have the versatility of using Threads or AsyncTasks for running background threads. This tool came around after the API 21 (Android Lollipop I think) and it's very optimized for battery usage
This also can be used for recurring jobs at set intervals.
Any software naturally runs code instructions one at a time, these ones are run in sequence. But modern software has the capability to take advantage of every aspect of the hardware specs, especially CPU's. Actually nowadays any computer without multiple CPUs it's pretty much considered old.
So, getting back to the first idea, once a task or instruction is executed, the CPU can't do anything else, this is called a blocking call.
This term takes place directly for computers with multiple CPUs (Cores) when the CPU it's able to generate multiple Threads that can be executed concurrently. That's the case for modern Android devices, almost all of them have multiple cores in its configuration.
The main Thread is used for UI related activities, this is mandatory, and another operations are handled or should be done with the help of the threading techniques, this is because the any demanding operation can freeze the UI, the same happens with network-related operations.