The judiciary is the only branch of the U.S. government that frequently restricts its own power. The constitutional system of checks and balances requires the courts to decide when Congress or the president is overreaching. But no one tells the U.S. Supreme Court justices, who are appointed for life, that they're exceeding their constitutional power (except other justices). The system depends on judges acting judiciously.
The Trump administration believes the system failed on Friday, when U.S. District Judge James Robart of Seattle granted a motion by the states of Washington and Minnesota to suspend enforcement of the president's week-old travel, immigration and refugee policy.
The Justice Department's appeal at the 9th U.S. Circuit Court of Appeals to stay Judge Robart's order is, fundamentally, a call on federal courts to limit their own authority to review the president's power to decide who comes into and goes out of this country.