What is Azure Firewall rule?

A rule belongs to a rule collection, and it specifies which traffic is allowed or denied in your network. They're the third unit to be processed by the firewall and they don't follow a priority order based on values. The processing logic for rules follows a top-down approach. All traffic that passes through the firewall is evaluated by the defined rules for an allow or deny match. If there's no rule that allows the traffic, then the traffic is denied by default.