A U-bolt is a bolt in the shape of the letter U with screw threads on both ends. Carbon steel U bolt clearly means the material of the U bolt is carbon steel.
Carbon steel U bolt has primarily been used to support pipework, pipes through which fluids and gasses pass. As such, carbon steel U-bolts were measured using pipe-work engineering speak. Carbon steel U-bolt would be described by the size of pipe it was supporting. Carbon steel U-bolt are also used to hold ropes together.