can you please help understand exactly what the following code means:
route-map MY-LOCAL-POLICY permit 10
match ip address 150
set ip next-hop 18.104.22.168
set interface Null0
Especially, what does the Null interface do?
The PBR matches the Datagrams recieved by the Interface and forward it based on the routing policy.
Your config means, If packet hits an interface and matches ACL 150 , then forward the packet and sets the next hop to be (22.214.171.124) , However, If the Next hop (75.x.x.x) is not reachable, then Set the next hop of the packet to (Null0 interface) , In other word black hole or drop the traffic.