join is an extension of the thread metaphor. When threads are spawned, these are said to have forked from their parent thread. To join threads means to weave these back together again.
In practice, join means wait for the other thread to finish. The join() function instructs the OS to defer scheduling the calling thread until the other thread finishes.
{
.join()//means guarantee - waiting for All Threads to Finish Using join Handles
. A JoinHandle is an owned value that, when we call the join method on it, will wait for its thread to finish
}