The US Supreme Court is simply blocking the Government from executing this parlor trick.
Once an individual is removed to a foreign country, US Courts lose jurisdiction to hear challenges. This decision prevents the Trump Administration from subverting the law.
Stated differently, Trump (aka Grand Wizard Stephen Miller) is claiming that the Aliens Enemies Act allows his Administration to bypass the normal protocols (due process) to remove individuals. The Supreme Court is checking that legal claim, until fully litigated.
Link: https://www.supremecourt.gov/opinions/24pdf/24a1007_g2bh.pdf