The term "Marxist humanism" has as its foundation Marx's conception of the "alienation of the labourer" as he advances it in his Economic and Philosophic Manuscripts of 1844--an alienation that is born of a capitalist system in which the worker no longer functions as (what Marx terms) a "Free Productive" being. And although many scholars consider late Marx less of a humanist than the Marx who wrote pre-Das Capital, as his later works are rather bereft of references to this alienation, the notion of alienation remains essential to Marx's holistic philosophy.
There is, as is assumed under the very notion of alienation, a human who, when disenfranchised by his own labour, somehow becomes less human--in fact, Marx says he becomes objectified. Man naturally produces for his own benefit--and, furthermore, he freely produces--however, under a capitalist society in which the means of production give rise to "fettering" productive relations--i.e. there is a single capitalist employing an army of workers at wages just sufficient to provide for their mere subsistence--the worker becomes a slave. He is no longer a free productive being, but instead he absolutely must produce to simply meet his most basic needs.
However, there is more to this than meets the eye. At one point in his Economic and Philosophic manuscripts, Marx writes: "A forcing-up of wages (disregarding all other difficulties, including the fact that it would only be by force, too, that the higher-wages, being an anomaly, could be maintained) would therefore be nothing but better payment for the slave, and would not conquer either for the worker or for labour their human status and dignity." It is here that Marx as a humanist is most evinced.
It is not the 12-hour work-day alone that enslaves man, forcing him to give his entire productive being--his natural skills, or that which constitutes is essence as a man--over to another. Rather, it is the very (philosophic) condition behind the capitalist structure that enslaves man--basically, capitalism is not conducive to democracy on Marx's conception. Capitalism inherently gives rise to an elite bourgeoisie for whom the rest of society--the proletariat--must work. Insofar as this is the case, the proletarian himself will never be able to dictate the conditions of his work, they will always be determined by the capitalist himself. So even if the capitalist pays the worker higher wages than he himself incurs (from his profits made off the labourer's efforts), he still controls the terms of the worker's production.
Insofar as man only has his LIBERTY to produce, and produce according to his own conceived ideas (i.e. he designs a very fancy shoe and wants to see this shoe materialize), capitalism, as a system, will be an eternal stymie to man's natural freedom. This is Marx's humanism.