TL;DR
대비마마 (daebi mama) = late king’s official wife
For those who are Sageuk enthusiasts:
I think you are talking about 대비마마 (Daebi mama). It’s a term used to address former/late king’s proper queen (not concubines). She is usually the mother of the current king, although not necessarily. Meaning, even if a concubine’s son ends up becoming the new king, only the late king’s official queen is still called Daebi. If her own son becomes the king, her title is 왕대비 (Wang daebi) but she will still be address as Daebi mama.
If her grandson becomes the king later, after her son passes away, then she is called 대왕대비 (Daewang Daebi).