Rebar is common steel reinforcing bar, an important component of reinforced concrete structures. It is usually formed from mild steel, and is given ridges for better frictional adhesion to the concrete.
A tied rebar beam cage.
Concrete is a material that is very strong in compression (pushing), but virtually without strength in tension (pulling). To compensate for this imbalance in concrete's behavior, rebar is formed into it to carry the tension loads.
Although rebar has ridges that bind it mechanically to the concrete with friction, it can still be pulled out of the concrete under high stresses, an occurrence that often precedes a larger-scale collapse of the structure. To prevent such a failure, rebar is either deeply embedded into adjacent structural members, or bent and hooked at the ends to lock it around the concrete and other rebars. This first approach increases the friction locking the bar into place while the second makes use of the high compressive strength of concrete.