A bead with a silicone filled inside, with a smaller hole through it. Also called slider beads or rubber filled beads or smart beads.
You can also use them as sliding clasps (vs a sliding pair of knots) by threading two lines through from opposite sides.
Lead-free. Nickel-free. 5.5mm x 3.8mm Hole size is approx. 2mm. By the pack of 2.