The 39th parallel south is like an imaginary line that goes around the Earth, just like a belt around your waist. It's located exactly 39 degrees south of the Earth's equator, which is like a big, invisible circle that goes around the middle of the Earth.
This line runs all the way around the Earth, but it's mostly important because it goes through a lot of different countries and states. In particular, it runs through parts of Uruguay, Argentina, Chile, Australia, and New Zealand.
When people talk about the 39th parallel south, they usually mean it in the context of these countries and states. They might talk about how the climate is affected by being on or near the line, or how the line affects trade or travel between different places.
So, while it might seem like just a boring line on a map, the 39th parallel south is actually an important way for people to talk about different parts of the world and how they're connected!