Back when the ACA was originally passed, it included an “individual mandate” which imposed a penalty on people for not having health insurance. Many argued that Congress did not have the constitutional authority to compel people to purchase health insurance. In 2012, the NFIB v. Sebelius case determined that such constitutional authority exists in the “tax and spend” clause that allows Congress power to levy taxes.
Then, in 2017 the Trump administration reduced the tax to $0, which then raised a question whether it could be called a tax. If not, it could no longer be propped up by the ruling in NFIB. Many argued that this action undercut the law’s legitimacy and rendered it unconstitutional. This was the central argument in California v. Texas, decided June 17th. Instead of providing a ruling on whether the ACA could stand as modified, the Supreme Court held that the plaintiffs lacked eligibility to bring the case, leaving the ACA intact but still vulnerable.
https://www.supremecourt.gov/opinions/20pdf/19-840_6jfm.pdf — full court opinion here