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.
Then followed the QR for the Yard, soon the Full set of QR become available.
The different combination of parrel beads is long and confusing (to a novice like me), so to make things simplier, follow the pdf link for the complete run down on my favourite accessory