It can generically refer to any powder used to cast a spell, especially if harmful in nature, but specifically refers to a concoction of natural ingredients that can be used to cause harm, trouble or even kill an enemy. Often it is substituted with graveyard dirt.
In practice, it was often used to create illness in victims, such as swelling of the legs or blindness. Recipes for making it vary, but primarily include graveyard dirt and snakeskin. Other ingredients may include ash, powdered sulfur, salt, powdered bones, and powdered insects.
As late as the 1930s, goofering was a regional synonym for voodooing, and in North Carolina at least, the meaning of the term was broadened beyond spells of damage, illness, and death to include love spells cast with dominating intent.
Another word for goofering is "poisoning," which in this context does not refer to a physical poison but to a physical agent that, through magical means, brings about an "unnatural illness" or the death of the victim.
Hoodoo and Voodoo can be subjective. Some think that the powers of a conjurer can only harm you if you believe, most however dismiss this theory. It is widely believed that the complete faith of the conjurer is required for the spell to work.
Even then, one must be careful what they wish for, lest karma rebounds onto you. A skilled practitioner would have safeguards to protect themselves from this possibility.
Check out Goofer Dust by Big Lucky Carter.