Being disconnected from your labour, and feeling disconnected from other human beings.
The central concept was alienation, but Marx saw the source of this alienation not as a problem of consciousness, as Hegel put it, but in the material conditions of capitalist society. Under capitalism, human beings led a nonhuman existence, being alienated from their work, from the product of their labor, from one another, from nature, from their own true selves. The solution was not in the realm of ideas, but in action to overturn these conditions.
Alienation refers to estrangement in some form or another.
Alienation comes from Hegel, which, for him, was the recognition of the other. Hegel believed that freedom could come from alienation.
Alienation was also seen for Hegel as the separation of man from God, and that the only way humankind can come to know itself is through the separation from God. This idea seems to have been derived from hermetics.