This just means where your filesystem find root of it. if you mount it to a /urs/bin means your root will be /usr/bin. anything before /usr or / is unaccessible. after mounting to /usr/bin you filesystem work as its root directory is /usr/bin

here is a detailed animated video about filesystem, though it just explain filesystem generally --- https://www.youtube.com/watch?v=HmyiHyfbZ0k&t=2s