I'm new with multi-threading and I need to get the whole idea about the "join" and do I need to join every thread in my application ?, and how does that work with multi-threading ?
no, you can detach one thread if you want it to leave it alone. If you start a thread, either you detach it or you join it before the program ends, otherwise this is undefined behaviour.
To know that a thread needs to be detached you need to ask yourself this question: "do I want the the thread to run after the program main function is finished?". Here are some examples:
When you do File/New you create a new thread and you detach it: the thread will be closed when the user closes the document Here you don't need to join the threads
When you do a Monte Carlo simulation, some distributed computing, or any Divide And Conquer type algorithms, you launch all the threads and you need to wait for all the results so that you can combine them. Here you explicitly need to join the thread before combining the results