They are referring to hypersonic glide vehicles here. Whereas conventional warheads follow a ballistic trajectory that can be predicted while in transit, hypers

They are referring to hypersonic glide vehicles here. Whereas conventional warheads follow a ballistic trajectory that can be predicted while in transit, hypersonic glide vehicles (HGVs) can at least to a limited extent maneuver. HGVs also tend to fly on a lower trajectory than ballistic warheads, meaning they are more difficult to detect by radar. These two factors make HGVs good at getting past current anti-ballistic missile (ABM) systems like those the US has spent a lot of time and money developing.