An acronym for the literary organization headed by Berlioz. Presumably the full version would be "Massovaia literatura," meaning "literarure for the masses." There was in fact no such organization.
In 1921 Bulgakov began working at the Lito (Literary Section) of Glavpolitprosvet (Head Office of Political Enlightenment).
In the late 20s there were several literary organizations that competed for official favor. In 1934 these were supplanted by the Soviet Writers' Union, first headed by Maxim Gorky. After 1934 one effectively had to be a member of the Writers' Union in order to work as a writer.