reserved constituency is Some of the weaker sections of the society cannot contest elections because of economic and financial problems. It is also because the richer and the more influential sections of the society overpower them and don't allow them to contest elections; and even if they do, the weaker sections are never able to win. So, the makers of our Constitution thought of upgrading of the society and thought that ALL the sections of our society MUST get a chance to contest and win elections to govern the country due to our democratic nature. So some of the constituencies are reserved for the SC (Scheduled Castes) and the ST (Scheduled Tribes) where only the weaker sections can participate and contest elections.
Hope I helped.