The olive is the small sacrificial ferrule that does the actual sealing in a PTFE hose end. It slides onto the exposed PTFE liner after the braid is eased back, and when the nipple is drawn in by the socket nut it squeezes the olive tight against the liner. Because it deforms in service, an olive should be treated as a one-use item and replaced whenever a fitting is stripped and rebuilt, which is exactly what this AN6 part is for. Fit it the correct way round with the liner fully seated against its internal shoulder, and check no stray braid strands are trapped beneath it, as that is the usual cause of a persistent weep.