A girder is any of the many beams used in buildings and bridges that provide support and actually hold them up.
If you've ever seen a building going up, you know the first they do is put up a steel or wood skeleton that will eventually take the weight of the roof, the siding, the floors and all the rooms inside. That skeleton is made of girders. Bridges also are made with girders, but they usually remain visible long after the steel beams in buildings have been covered by glass, brick and wood.
