What is this "Shared" in Finder's Sidebar?
System preferences > Sharing

You can choose what to share and with what conditions. You can share files with other devices connected to your network, but they have to know your UserID and password, you can share printers and many other functionalities.

Obviously, you can choose to enable the sharing or not just checking the "services" on the left sidebar. If you disable everything, your "shared" folder will not be accessible.