The term thread is shorthand for "thread of control". A thread is the path taken by a program while running, the steps performed, and the order in which the steps are performed. A thread runs code from its starting location in an ordered, predefined sequence for a given set of inputs.
The use of threads within a job allows many things to be done at once. For example, while a job is processing, a thread may retrieve and calculate data needed by the job to finish processing
Every active job has at least one thread, which is called an initial thread. The initial thread is created as part of starting the job. In the threads on iSeries™ Navigator, by default, you will see Initial as the type of the first thread in the list. The initial thread is the first thread created within the job when it starts.