Ableism is the cultural, unconscious valuation of ability. It manifests as discrimination against those whose abilities are less than the normative ability of the "able-bodied" and those of "able-mind." Those who are considered less able or "disabled" are marginalized and othered. They become the minimum standard by which the "able" demonstrate their ability and reinforce the culturally dominant definition of normal and ability. Like any other relation of power between races, sexes, and those from different socio-economic statuses - ableism perpetuates "ability" as good, status-quo, and valued by devaluing its invisible, voiceless opposite - disability.
I think Xiaoyu's comment under the second image on her post about "invisible discrimination" is illustrative. African-Americans have been discriminated against and criticized for having inferior intelligence. Women have been discriminated against and criticized because of their frail minds and bodies. Both modes of prejudice highlight the "invisible discrimination" that people with disabilities face without ever mentioning disability. This is the direct product of ableism.