choke tubes
Sounds like your gun is threaded at the muzzle for "choke tubes". Choke tubes are removable/interchangeable tubes that install and remove at the muzzle, typically with a special wrench. The tube will usually be stamped as to its type, on the side, visible once removed from the barrel.
Be advised that if not maintained well, a shotgun with interchangeable chokes can have the tube stuck/rusted/corroded in the muzzle and be a booger- bear to get out.
Some you tube searching or a visit to a friendly shop should get you up to speed. Good luck with it.