It’s a tube magazine attachment for pump and semiauto shotguns that adds up to 26+1 capacity. It’s pretty pricey, though, sometimes twice as much as the gun itself.
For Remington 870s you can get up to 22+1: https://www.xrailbyrci.com/mm5/merchant.mvc?Session_ID=dbff0ac43a57d376254e328df93bac1b&Screen=PROD&Store_Code=xrail&Product_Code=1103-0001FRB&Category_Code=xrail_remington