They’re a set of beads of varying diameter (I’ve seen everything from less than a quarter of an inch, to over two inches) that are on a STRONG string (often surgical steel cabling). The beads are pushed into the anus (i.e. several of the beads are inserted). Then, as orgasm approaches, your lover pulls the beads out at varying speeds (some people like a quick tug of all of them, others like each bead to be slowly removed). For a woman in particular, this can be extremely enjoyable.