Short answers, pulled from the story.
Congress leaders, including Gandhi, were in jail for participating in the Civil Disobedience Movement. The boycott effectively crippled the first conference, since Congress was the dominant political party in India and its absence left no way to reach a broadly legitimate agreement.
The conferences were held at the recommendation of Muhammad Ali Jinnah to Viceroy Lord Irwin and Prime Minister Ramsay MacDonald. The Simon Commission's report, submitted in May 1930, also provided the formal basis for calling the conferences.
The Gandhi-Irwin Pact of 1931 was an agreement under which Gandhi and other Congress leaders were released from prison. In exchange, Congress agreed to participate in the second Round Table Conference, making it the diplomatic step that brought the independence movement's most prominent voice to London.
Ambedkar, representing the Depressed Classes, argued for a separate electorate for Untouchables on the grounds that they constituted a distinct political group needing independent representation. This placed him in direct conflict with Gandhi, who insisted the Untouchables were Hindus and should not be treated as a separate minority.
The conferences fed into a Joint Committee process that produced the Government of India Act 1935. Under Sir Samuel Hoare's supervision, the work carried out between September 1931 and March 1933 resulted in legislation that restructured India's constitutional arrangements and served as the direct predecessor to independence.
The third session in November-December 1932 attracted only 46 delegates because both the Labour Party and the Indian National Congress refused to attend, and most major Indian political figures were absent. Without those key blocs, the proceedings had little authority and produced no new agreements.