"Payment? ...You've got to give the door something?"
"Yes ...Blood, if I am not much mistaken."
-- Harry and Dumbledore in the sea cave (HBP26)
A magical effect where a sacrifice of blood is required to open a portal. A wizard seeking passage must offer some of his own blood on the portal to make it open (HBP26).
References from the canon
Voldemort used blood magic to seal the entrance to the underground chamber where he hid his locket Horcrux. Dumbledore cut his own hand to produce the blood needed and sprinkled it on the rocks (HBP26).