What is Freight Breakdown?

Not knowing your mode of transportation it could have several definitions. Without the context I believe this is the answer you are looking for.

Basically it means unloading it from the consolidated transportation mode back into a single shipment.

In LTL - shipment would be in a consolidated trailer with lots of other shipments going the same direction as yours but when the trailer gets downline it needs "broke down" and sorted to different truck.

Airfreight - Freight forwarders put multiple shipments into consolidations as with a trailer on LTL, Just smaller version, and shipments have to be unloaded and sorted.

Ocean freight LCL, pretty much the same process but given most shipments in this mode are international, the container is emptied into a secured warehouse to wait for each individual customer to get his shipment through customs and out of the warehouse.