A person who is able to find things when no one else is able to find them. A scavenger in the truest sense of the word that usually attacks in pairs. This came from the tradition that the jackal will sometimes lead a lion to its prey.
Oppositely, the term can also be used in a negative connotation to describe someone who relentlessly asks you for something that is in your possession and takes it without offering anything in return.
Person #1: Do you have any blow?
Person #2: No man I couldn't find any. Let's call Jason, he's a jackal, he'll know where we can get some.
Person #1 walks into party Jason: Hey man you got any blow I can have?
Person #1: Get away from me you fucking Jackal I just got here.