>>1877443>>1877444Looks like a hose clamp olive. They're used for clamping down into a tight seal between a brake hose and the brake lever assembly or caliper.
Don't know what it's doing in that bleed kit.
Generally I don't know what most of the gubbins in that kit is for. For my Shimano brakes all I need is the cup and the hose to do a gravity bleed.