Special:MyPage is a special link that is dynamically linked to the current user's userpage. So, whoever clicks on it will be taken to their own user page.
If you add a subpage to the link, then only you will be taken to your subpage of that name. Other users will be taken to the subpage of that name in their namespace (if it exists).
This makes the link "private" by only letting the correct user get to the page via that link. Other users will diverted to their own userspace.
The code [[Special:MyPage/To-do list|To-do list]] creates this: To-do list.
However, this only makes the link private, not the page itself. Others can still get to it via the search box or by creating a new link with its entire title.
