Policy-based routing is a way to control which path traffic takes on computer networks. Imagine that a network consists of many roads, with each road leading to a different destination. Policy-based routing lets you decide which roads your traffic should take.
For example, if you wanted traffic from a certain group of computers to take the fastest route to their destination, you could create a policy-based routing rule that says, "traffic from this group of computers should take the fastest road to its destination." Then, whenever traffic from those computers needs to go somewhere, it will take the fastest route to get there.