In his chapter in the Cambridge Companion to Adorno, entitled 'Negative Dialectic As Fate' J.M. Bernstein gives us a good interpretation of the core thesis of Horkheimer and Adorno's Dialectic of Enlightenment.
He states:
Dialectic of Enlightenment is the attempt to provide a conceptual analysis of how it is possible that the rational process of enlightenment which was intended to secure freedom from fear and human sovereignty could turn into forms of political, social and ultural dominaton in which humans are deprived of their individuality and society is generally emptied of human meaning.