Concrete, although strong in compression, is weak in tension. For this reason it needs help in resisting tensile stresses caused by bending forces from applied loads which would result in cracking and ultimately failure.
Consider a beam supported at each end and carrying a load. Under this load, the concrete in the top layers will be compressed (shortened), but those in the bottom layer will be tensioned (stretched). As the load increases the beam deflects and cracks will occur in the bottom face and it will eventually fail as the concrete is weak in tensile strength.
There are two ways to resist this low tensile strength - by using reinforcement or by pre-stressing. Reinforcement in the form of fabric or profiled bar (typically steel but may be FRP) of differing gauge or size, or potentially specialist fibres depending on the application, is placed in areas where tensile stresses will occur under load. Pre-stressing is concrete where an internal stress has been introduced through steel strand impart compression into the concrete.
Reinforced concrete is strong in tension and is able to absorb the stresses developing in the concrete so long as a good bond between the concrete and reinforcement or pre-stressing strand is maintained. By limiting the stresses that the steel has to deal with, the cracking in to concrete can be kept within acceptable limits. Concrete used in most construction work is reinforced.