A Federal court ruled, during the Obama administration, that the payments Obama was making to the insurance companies to help pay for the insurance companies' requirement to subsidize co-pays and deductibles for some Obamacare insured were illegal as Congress had not appropriated the money to pay for it.  The judge said the payments could continue, however, at the administration's discretion while they appealed the ruling.  They never appealed the ruling, they just continued the illegal payments to the insurance companies.
So Trump decides to respect the judge's ruling, just like he did on his immigration plan, and discontinues the payments - to the insurance companies.  Yes, the ball is now in Congress' court, as it should be when it comes to appropriating money.