If you're one of those people who thinks they're too good for ABBA and doesn't want joy in their life and you haven't seen the movies, the first is about Sophie (Amanda Seyfried) trying to figure out which of three men knocked up her mom Donna (Streep) back in the day so he can walk her down the aisle.
The second movie is about Sophie dealing with Donna's death, plus flashbacks of Young Donna (Lily James) hooking up with her three paramours. In the second, we learn that Donna is recently deceased, and her mother, played by Cher, drops by for a visit.
Now, here's where Mamma Mia! Lore comes into play. In the first movie, Donna makes a reference to her mother being dead, and whole you could write this off as a figure of speech due to their estrangement, or just a plot hole when her mom then appears, very much alive, there's another explanation: people can come back to life in this universe.
ABBA released a new album in 2021, and it's good. Anything is possible.