Question

what are reserve constituency and why do reservation is necessary

Answer 1

The Indian Constitution provides for a special system of reserved constituencies for the weaker sections of the society. ... In a SC reserved constituency only someone who belongs to the Scheduled Castes can stand for election.

Reservation in India is the process of allocating some percentage of seats (vacancies) in government institutions like educational institutions, jobs for members of backward and under-represented communities (defined primarily by caste and tribe).
Thus reservations are anti-thesis of development and equality. We don't need reservations based on castes or religion but only to actually provide aid to those who have minimal resources; and merit should be given equal and due importance in admission procedures as well employment opportunities.

hope it clarifies ur doubt dear friend...