What is a separate electorate, and why did he want one?
A separate electorate is a voting arrangement in which a named community has its own roll and elects its own representatives, and only members of that community vote for those seats. It is to be distinguished from a reserved seat inside a joint electorate, where a seat is set aside for the community but the whole electorate — the majority included — casts the votes that fill it. The difference is who does the choosing, and Ambedkar thought it decisive.
He wanted separate electorates for the Depressed Classes because he believed a representative chosen by the majority's votes would answer to the majority, not to the community he was supposed to speak for. A candidate acceptable to caste Hindus would win the reserved seat, and the truly independent voice of the Untouchables would be filtered out before it could be heard. Muslims had been granted separate electorates decades earlier; he argued his people, uniquely vulnerable to the majority around them, needed the same protection.
The demand was granted in the Communal Award of 1932 and then taken back under pressure. Gandhi, opposed to any arrangement that set the Untouchables apart from the Hindu fold, fasted against it, and the Poona Pact that ended the fast replaced separate electorates with a larger number of reserved seats inside the joint electorate. Ambedkar signed to save Gandhi's life and spent the rest of his career arguing he had been made to trade the one thing that mattered — the independent vote — for a larger number of seats filled by everyone else's.