Okay kiddo, so have you ever been to a birthday party where all the kids were the same age as you? Well, that's kind of like a demographic, which is just a way of saying a group of people who share certain qualities.
Now, imagine if all of your friends started having babies at the same time and all those babies grew up together. They would be another demographic, but one that is younger than yours.
A demographic threat is when one demographic group has a lot more people than another group, and that can cause some problems. For example, if there are way more older people than younger people in a country, there might not be enough workers to support them all.
In some cases, a demographic threat can happen because of things like war or disease that kill off a lot of people in one group. In other cases, it can happen because people in one group are having a lot more babies than people in another group.
So, just like you can have too many toys and not enough room to play with them all, a demographic threat is when there are too many people in one group and not enough resources to go around.