Okay kiddo, so you know how some grown-ups like to do what's called "sexy stuff" with another grown-up they love or care about? Well, sometimes a grown-up might not feel like doing those things as much as they used to or even at all. This is called hypoactive sexual desire disorder, but we can call it HSDD for short.
Basically, HSDD means that a grown-up doesn't have as much interest or desire for "sexy stuff" as they used to. It's kind of like if you had a favorite toy that you used to play with all the time, but now you don't want to play with it as much. Sometimes, HSDD goes away on its own, but other times a grown-up might need to talk to a doctor to figure out why they don't feel like doing "sexy stuff" and see if there's a way to help them feel more interested again.
It's important to remember that there's nothing wrong with a grown-up if they have HSDD, and it doesn't mean they don't love their partner or that their partner isn't attractive. It's just a feeling, like sometimes you might not want to eat your favorite food even though you still like it. And, it's important to always respect a grown-up's feelings and wishes, even if they don't want to do "sexy stuff" as often as they used to.