How do left over materials in space pose a threat to people on the ground?

Leftover materials in space, such as defunct satellites or fragments from collisions, pose a threat to people on the ground because they can re-enter the Earth's atmosphere unpredictably and potentially cause damage or injury upon impact. There is a risk that these objects could land in populated areas, posing a danger to human life and property. Efforts are made to track and monitor space debris to mitigate this risk.