The threads window is displayed using View|Threads. The threads window lists the active threads of the program.
The contents of the threads window are updated every time the program is stopped by the debugger. (That is, the window does not reflect the state while the program is running.)
The first column shows the thread ID, the second column identifies the location where the thread currently executes.
The marker in front of the line tells which thread currently is active:
By clicking a listed thread, the active thread is switched, and the corresponding windows are updated. In particular, the source window displays the location where the active thread is currently halted.