It’s either gehena or shaol, I don’t remember which. It is the place for the dead, before the gates of heaven were opened by Christ. I think it’s kind of like purgatory, except even after purging the attachment to sin, one couldn’t move into heaven as the gates had not yet been opened.
It was here to which Christ descended after his crucifixion, and he brought forward those who were deserving of salvation, according to their particular judgement.
I tried looking this up in the CCC, but couldn’t find it. Perhaps someone else has a better answer for you.
CARose