Concrete is very strong in compression (loads trying to crush the concrete,) but very weak in tension (loads that try to pull the concrete apart,) and very weak in shear (loads that try to snap the concrete like how you’d break a twig in half.)
Steel is a linear elastic material that is basically equally strong in all directions – tension, compression, shear… has pretty much equal resistance to all of them.
By putting steel inside of concrete, specifically in the parts of the concrete that will be subjected to tensile or shear stress, those stresses are carried by the steel instead of the concrete. It allows you to use less steel than if you were making buildings entirely out of steel (Concrete is much cheaper than steel,) but it allows you to compensate for the areas where the concrete is weak.
