The Mark of the Huntsclan is a "birthmark" of the Huntsclan.
All Huntsclan members are born with a birthmark in the shape of a coiled green dragon on their body. It is shown that any baby born with this mark will taken and raised by the Huntsclan, as was Rose. Huntsclan members were undercover at hospitals for infants with the mark and kidnapping such children. It is not proven if it grants the bearer any magical powers.
As Rose stated, the mark means all members were born to slay dragons. It is usually found on their right palm and arm. However the Huntsman's dragon birthmark is round his chest and face on the left hand side.