RCC is a combination of concrete & steel bars. Concrete is strong in compression but is weak in tension.
First let us have a look at the basic definitions of compression & tension.:
Now you are sitting on a chair.The force in the legs of the chairs due to your weight is known as the compression. If you carry a bucket of water in your left hand , it is said that this hand is carrying tension which is equal to the weight of bucket & water.
In building elements there are cases when in a single element there could be compressionononesideandtensionontheotherside.
Since concrete is weak in tension, it needs to be strengthened in this zone where tension is acting. Since steel is excellent in tension , steel bars are provided in the tension zone of concrete.