The parrel bead is sometimes referred to as a parral bead or even a parallel bead, but they all describe a set of beads threaded onto a rope used to allow a sail or yard to be attached to a mast & slide up & down.
They were traditionally of wood & varnished ash on bigger boats. Varnishing small beads is fiddly, so was rarely done, leaving them prone to attract mildew, soften & split. We have ours made specially for us in black synthetic, which is durable, clean & long lasting.
In the Drascombe context, a standard set for the gunter yawls is 4 strings, one of three beads & three of four beads, all on plain line.
This is followed by a Quick Release (QR) set for the Yard.