This was announced via the in-game message of the day. This is where Fornite displays important messages daily.
This item was leaked in the v10.10 patch notes as well as the Week 4 loading screen. There is no information currently on how the Junk Rift works, but we were able to see a possible glimpse from the Week 4 loading screen. It appears that when you throw the Junk Rift, random items appear out of the sky!