Let's say you're a trucking company named Apple Trucking, talking on the phone with T.Q.L. (Total Quality Logistics, one of the world's biggest freight brokers). You accept a load to run from Atlanta, GA to San Antonio, TX, and T.Q.L. agrees to pay you $3,000 to move it. A "double brokered load" is when Apple Trucking takes this load and re-sells it on the open market to another carrier.
Say Apple Trucking re-sells this load to a different carrier called Success Trucking for $2,800. This load has now been double-brokered, because it went from T.Q.L. → Apple Trucking → Success Trucking — Apple Trucking has "re-brokered" it out to a carrier T.Q.L. knows absolutely nothing about.
Double-brokering is technically something you can do, but across the industry it's a big no-no. If the original broker catches you, they'll blacklist you — marking you on a do-not-use list, preventing any future loads with their brokerage. If you're running a load with a big-name broker like T.Q.L. and get caught, you've killed a relationship that could have given you tons of loads, and it becomes harder to find well-paying work going forward. I'd encourage you to never double broker a load and risk this happening to your business.