It's been a while since I pulled the barrel on my dads 97 (now my brothers) but I think they are referring to what a machinist would call interrupted threads. The threads cover half the circumference of the barrel in two quarter circumference sections. You can slid the barrel almost all the way home then give it a 1/4 turn to tighten the threads. Not unlike the breach of a howitzer.

Like I said though, my memory on this may be faulty or I may be thinking of another shotgun. Several makers used this system.