Okay kiddo, so a settlement is a place where people live. When we talk about southernmost settlements, we mean the settlements that are the farthest south. This means that they are closest to the South Pole which is a really cold place.
When people live in a really cold place like the southernmost settlements, they have to find ways to keep warm. They need warm clothing and a way to heat their homes. They may also have to find ways to get food because it's not always easy to grow plants when it is so cold.
The southernmost settlements are not very common because it's hard to live in such a cold place. People who choose to live there are usually scientists, researchers, or other people who have a specific reason to be there. They may be studying the environment or trying to discover new things about the South Pole.
So that's what southernmost settlements are, kiddo. They are just places where people live that are really, really far south and it's cold there.