>>507528839It means they fly very high to limit drag and fly in almost perfectly straight lines. When they dip down deeper into the atmosphere they lose what little maneuverability they had. They are hard to intercept because they have limited engagement windows during mid course. On terminal approach they are a lot easier to engage but you need defenses right on top of the target to be effective.
Hypersonic missiles are an attempt to counter strong missile defenses in a cheaper manner than saturation, stealth or using nuclear weapons. Those methods was how the USSR was planning to counter the US missile defenses and so long as your enemy doesn't have lots of counter missile systems it works well enough to be better than other means.