I don't know what this "Shared" is in the Finder's Sidebar. Can these other machines access any of my information? Should I turn this off?
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.