Joanna
The meaning is found in the term "self-estrangement." Marx has taken from Hegel the idea of the human essence as "freedom." The meaning of "freedom" here is the developed capacity to perceive truly including the developed capacity to perceive values truly and on this basis make actual life "good." The ethical essence of a good life is relations of "mutual recognition" - "love."
Human history is understood as an "educational" process through which this capacity develops. Prior to its full development we are "estranged" from it. Since it constitutes our "essence," these prior states are states of "self-estrangement." They are also states in which perception involves illusion to some significant degree, illusion from which it is very difficult to escape.
Like all other "being," human being is understood to be active being. This ontological idea underpins the epistemological doctrine of "praxis"; the doctrine that explains how human experience provides a basis for true perception.
Partly for this reason, both the capacity to perceive truly and understanding based on it develop within the labour process. The model here is Hegel's account of the effect of slavery on human consciousness and understanding. The master/slave relation, like the capitalist/wage laborer relation, is a relation of self-estrangement which, while producing illusions, also eventually produces both an increased capacity for understanding and increased understanding. This includes an increased capacity to understand these relations as self-estrangement and based on this to refashion them to make them consistent with the human "essence" and the realization of the "realm of freedom."
This treatment of human being is based on ontological - "metaphysical" - premises different from those that have dominated "science" since the 17th century. Among other things, the latter have no logical space for the idea of human being as a being able to perceive truly.
The ground for these claims are "phenomenological" in the sense of Husserl, i.e. the ground is direct experience - the "intentionality" of consciousness - freed from interpretation and illusion (such as that present in the passage from Hume I quoted recently).