Used to describe the most extreme activities or members of a sub-culture. For instance, a hardcore hacker may be one who eats, breathes and sleeps coding - someone who does it "harder" than the majority of the sub-culture.
What exactly defines the "hardcore" part of a sub-culture is determined by the members of the sub-culture itself. Sometimes hardcore members or activities are frowned upon by the rest of the sub-culture or wider community as being too extreme, liberal or dangerous. This can damage the sub-cultue's reputation in the wider community by changing the mainstream perception of the sub-culture's identity.
On the other hand, sometimes hardcore members or activities are praised by the rest of the community as positive examples or role-models. A heavy, intense deep interest or passion in some activity can be described as hardcore.