Ok kiddo, so sometimes when a lady is pregnant, the baby grows inside her tummy and she gives birth to it when it's ready. But sometimes, the baby doesn't come out like it's supposed to. Instead, the baby stops growing and the body tries to protect the mommy by turning the baby into a hard rock-like thing called lithopedion.
This happens very rarely, but sometimes the mommy doesn't even know she's still carrying a baby because it doesn't grow anymore and it gets stuck in her tummy. It's kind of like how a snail puts on a shell to protect itself, but the mommy's body puts on a rock to protect her.
Usually, the mommy will feel some pain in her tummy and the doctors will find out that there's a lithopedion inside her. It's not dangerous to the mommy, but the doctors might need to take it out if it's causing her discomfort.
So that's lithopedion - a hard rock-like thing that forms inside a mommy's tummy when a baby stops growing, and it's there to protect her even if it can't be born like a normal baby.