What is the significance of Thread.Join in C#?

Join() is basically while(thread.running){}

{
  thread.start()
  stuff you want to do while the other thread is busy doing its own thing concurrently
  thread.join()
  you won't get here until thread has terminated.
} 

int fibsum = 1;

Thread t = new Thread(o =>
                          {
                              for (int i = 1; i < 20; i++)
                              {
                                  fibsum += fibsum;
                              }
                          });

t.Start();
t.Join(); // if you comment this line, the WriteLine will execute 
          // before the thread finishes and the result will be wrong
Console.WriteLine(fibsum);