Public Safety Announcement:
0.0.0.0/0 should be used sparingly. It means any host on any IP address (or any IPv4 address to be precise) on the Internet can use this route to connect to things on the other side. That means any host in this subnet can contact any host on the Internet and vice versa.
Here's a random sampling of traffic that hit my Firebox Cloud as soon as I set it up on the Internet, and why you might want to open up this type of traffic sparingly. As soon as you do this various nefarious (and accidental) traffic will start hitting any host with access from the Internet.
If you do open up to the Internet you might want security appliances inspecting traffic to and from your hosts prevent malicious traffic from accessing them. For example a WatchGuard Firebox would have prevented anyone using it from being infected by the WannaCry virus if configured properly. You can also use NACLs and Security Groups to limit access as will be noted in upcoming blog posts.
