When you driving down the road looking at all the semis driving down the road next to you, you may wonder how what they are hauling is loaded safely onto the trailer safely, especially if it's something like a huge piece of equipment that looks bulky and awkward. The answer to that is that everything has been balanced and rigged very carefully.
Balancing a load means that it has been loaded into the trailer so that the weight is as evenly distributed from front to back as possible. That includes making sure that the heavy items are where they will get the most support. The reason to do that is so that the truck can pull the trailer easily. If all the weight were at the front, it would be hard to even get the trailer attached to the truck and if it were all at the back of the trailer, then the trailer would drag. Putting the weight all at one side or the other would mean that the trailer would be at risk of tipping every time the trailer turned in that direction. So making sure that all the weight is evenly distributed and that the heaviest objects are in the center means that the trailer will ride smoothly and everyone will be safer on the road.
The term rigging comes from days when shipping was done on the tall ships. All the ropes and cables on the ship are called rigging. When the ships were being loaded, the riggers, who looked after the rigging on the boat, would strap down the cargo so that the boat was balanced. They would also strap down the cargo when the it was being transferred to and loaded on the wagons.
These days, the people who strap down the loads are still called riggers. They generally aren't using ropes because there are better options to be used, like canvas and nylon straps, but the riggers still have the final say as to if the load is secured well enough to go down the road. This is especially true when it comes to open loads like heavy equipment. Since there are no sides to make sure that the load doesn't fall out, it has to be firmly secured so that it can't fall off.
The people who do that balancing and the rigging of each load is what keeps the semi drivers safe and keep you safe as you are driving down the road next to them.