A flat deck trailer is a big, flat platform on wheels that can be pulled by a truck or tractor. It's like a big, wide, empty space for carrying things, like a giant skateboard without any sides. This makes it easier to load and unload big items like cars, construction equipment, or stacks of materials.
Just like when you play with your toy cars on a flat surface, the flat deck trailer is very useful for moving things around smoothly and without obstacles. The flat surface also means there's less risk of objects tipping over, because there are no sides to get in the way.
Sometimes, these trailers have rails or stakes around the edges to keep things from falling off, but most of the time it's just a big open platform. In any case, it's a simple but useful tool for transportation and moving things around.