At one time it referred to someone who believed that “white” was a distinct sub-species (rather than a pigmentation) consisting of European whites but excluding non-European whites (e.g. Jews, Arabs, East Asians etc.) and who also believe that the “white race” was superior to any other. A variant of this was the Nazi movement which tried to link the Indo-European (Aryan) language family to a purported “white race”. Note that many ethnic groups have “white” skin but were excluded for cultural or political reasons.
Among Democrats, it means anyone who disagrees with them.