Thread running and thread object

While learning basic thread management, I found difficulty in understanding these lines (in bold) from a book.

Once you’ve started your thread, you need to explicitly decide whether to wait for it to finish (by joining with it—see section 2.1.2) or leave it to run on its own (by detaching it—see section 2.1.3). If you don’t decide before the std::thread object is destroyed, then your program is terminated (the std::thread destructor calls std::terminate()). It’s therefore imperative that you ensure that the thread is correctly joined or detached, even in the presence of exceptions. See section 2.1.3 for a technique to handle this scenario. Note that you only have to make this decision before the std::thread object is destroyed—the thread itself may well have finished long before you join with it or detach it, and if you detach it, then the thread may continue running long after the std::thread object is destroyed.

When does a thread run even after the thread object is destroyed? Anyone have sample code or any reference?