To make a note so you don't forget something. It was a saying back in the day before cell phones and computers. When people would use wood pencil on paper.
Person 1: Did you get the directions to your cousin's place up north?
Person 2: Yeah.
Person 1: Jot da woo down before you forget it.
Girl: Did you bring your list?
Boy: Yeah, I jot da woo down.