What is policy routing?

From PFSenseDocs

Policy routing in pfSense refers to the capability of routing traffic by matching it to specific firewall rules. Each firewall rule allows you to select a gateway. If none is selected, traffic goes out your WAN interface. If you have any additional WAN interfaces (OPT WAN), or gateway load balancing or failover pools, you can select these in the Gateway field when adding or editing rules to direct matching traffic as you desire.