Dear Vishwanath
Ready Mix Concrete, popularly known as RMC refers to a concrete mix in unhardened state, manufactured especially for construction site. In traditional construction works, each constituents of concrete mix is mixed in specified proportions at site by the labor which is not always accurately mixes as the process is manual. However RMC is mixed mechanically so is more accurate mixing. See the article below for more details.
http://www.gharexpert.com/articles/Cement-Concrete-2006/Ready-Mix-Concrete_0.aspx