Explanation
https://cloud.google.com/network-intelligence-center/docs/firewall-insights/concepts/overview#shadowed-firewa Firewall Insights analyzes your firewall rules to detect firewall rules that are shadowed by other rules. A shadowed rule is a firewall rule that has all of its relevant attributes, such as its IP address and port ranges, overlapped by attributes from one or more rules with higher or equal priority, called shadowing rules.