This means that the queue you are creating will queue packets using the "default" queue.
The default queue is a 'pfifo' queue that can take a maximum of 50 packets.
You can see what queue types are available and create your own under
Thanks omega-00 for your more specific answer this time. You'll see, I'm learning RouterOS by my own and some topics (like the packet flow diagrams you mention in another post) are still just too cryptic for me. I appreciate your help.