What is a Marxist reading of a text?

Class.

How is it dealt with? What power relationships are at play?

From what I remember there's a handful (probably more) perspectives you can use to critique a text- feminism, Marxism, the literary canon, ecological and post colonialism. To look at something through a Marxist perspective would mean to consider the class differences and the effect of one on the other. Keywords such as proletariat, exploitation etc would be expected I guess. For example, the sentence (completely made up) 'He beat her as she did not complete her work' could be seen through both a feminist perspective and a Marxist perspective (although Marxism doesn't really deal with gender, there are power dynamics here and he's automatically set up a hierarchy in which the woman is expected to complete all the work or be beat and he is the one administering the beatings).

A marxist reading of a text is the only true reading of a text.