Basically, what this means is the set of ideas that causes people who are in a subordinate class to believe that their subordination is natural. This is an idea that is heavily influenced by Marxist thinking.
Marxists argue that dominant classes use ideologies to create hegemony over subordinate classes. Ideologies, in this sense, are ways of justifying the existing power structure in a society. Hegemony is the situation in which the subordinate class believes in the ideology and therefore believes that it is right and natural for them to be subordinated.
So, this ideology is essentially a deception played on the subordinate classes in order to make them accept being dominated by the dominant class.