The release is removing the capacity of a certain structural member to support forces like bending moment, torsion (more of torsion in the next posts) axial and shear forces. But most often release means release of the bending moment. Another simplistic explanation is to try to imagine a fully restrained or a fixed-fixed single span beam. Once you “release” the beam, there will no longer be negative bending moments at the supports such that it will approximate that of a simply supported beam. In a monolithic concrete structure, simply supported beams do not really exist. That is, it can still carry a bending moment based on the minimum flexural capacity of the section, that is if you provided the minimum requirement based from the code. The right term would be “fixed, but only to a certain degree."

 Need for the Release 

1. In order not to transfer the end moments on purpose. Sometimes the purpose is to avoid the congestion of reinforcement in the primary beam at the intersection of the secondary beam.
   
2. The development length is not enough. In order for the connection to qualify as a fixed connection, the bars should be able to fully develop in tension. 
   
3. The support cannot carry the end moments coming from the beam. So we release them, releasing does not mean that the Bending moment that we are releasing will goo out of the structure, instead it from the End to the Center of the beam.