Seems to be missing a word. They will jettison the hot-stage ring which connects the lower stage to the upper stage. It's a special piece required by hot-staging, where the upper stage is lit when it is still attached to the hot-stage ring on the lower stage (perhaps the ring is actually called the "hot stage"). The hot-staging ring is probably quite reinforced and therefore relatively heavy. I'm not sure why they want to jettison it though. The whole rocket is aimed to be nearly 100% reusable, so this is probably a temporary measure.
