Rebar (reinforcing steel)
Deformed steel bar embedded in concrete to carry tension, which concrete itself resists poorly. The ribs on the bar transfer force between steel and concrete.
What does rebar (reinforcing steel) mean?
Concrete and steel work together because they expand at nearly the same rate with temperature and because the alkaline concrete environment protects the steel from corrosion. Rebar does not prevent cracks from forming; it holds them tight and keeps the member working after they do. Its effectiveness depends entirely on being in the right position with adequate concrete cover.
In practice
Rebar lying on the ground and pulled up with a hook during the pour ends up in the wrong position more often than not, which is why chairs exist.