Old South Arabian refers to a group of ancient languages spoken in the southern part of the Arabian Peninsula thousands of years ago. It was used by people living in what is now Yemen and Oman.
Just like how you speak English or another language, people back then had their own language too. And Old South Arabian is the name for their language.
They used it to communicate with each other, to write things down, and to tell stories about their lives and cultures. And even though it's not spoken anymore, we can still learn a lot about their history and how they lived from Old South Arabian writings on rocks, stones, and inscriptions.
So, Old South Arabian is like the way people used to talk and write a really long time ago in a place far away from here.