The Supreme Court has halted a lower court ruling that granted the Committee on Oversight and Reform access to President Trump’s tax returns.  However, that’s not necessarily the lede.
The issue at stake is whether the legislative branch can penetrate the constitutional firewall which exists within the separation of powers. The House issued a subpoena in February for eight years of the president’s tax returns, which the Committee then later argued was part of the September House impeachment investigation.

All of the surrounding court rulings are predicated on accepting a constitutional process for an official impeachment investigation is underway.  However, the Supreme Court will hear arguments that will likely challenge that assertion.  To wit, within the buried lede to the background issue we find this paragraph:

On Monday, Douglas Letter, general counsel for the House Committee on Oversight and Reform, had sent a letter to the court, agreeing to a brief 10-day stay while the parties filed their court papers debating the need for an injunction while the case is being considered. (link)

The general counsel for the House, Doug Letter, knows that a SCOTUS ruling against the House endeavor could severely damage the legal and constitutional framework of the entire impeachment enterprise.  Therefore it is in Speaker Pelosi’s best interests to instruct Doug Letter to defer or stall a SCOTUS review until the impeachment crew has framed their partisan impeachment process.
Remember, the Supreme Court has not yet ruled on any ancillary case that touches upon the validity of the House impeachment process.
There is an important granular aspect to the validity of the House impeachment process that few are paying attention to.  The Supreme Court has not ruled on any case that touches the impeachment “inquiry”.  The House of Representatives does not want the Supreme Court to hear any case that touches on the impeachment “inquiry”.
In addition to the Trump Tax Case, there is another case from the HJC where they are attempting to use the framework of a constitutional impeachment process as the underlying authority for their endeavors.  That case surrounds the Grand Jury (6e) material from the Mueller investigation.
If the House loses the Tax case in SCOTUS or the HJC case in the DC Appellate Court, or SCOTUS, it means there is no constitutional foundation recognized to the “impeachment inquiry.”
Without the constitutional recognition of the judicial branch then: (a) Pelosi/Lawfare have to restart the process with a genuine House vote; or (b) the ongoing impeachment process will have no recognized constitutional standing; and (c) the Senate could ignore any House impeachment vote, cast without recognized constitutional standing.
That is why Nancy Pelosi and Doug Letter do not want SCOTUS to weigh in.

BACKSTORY to HJC case: On October 25th DC Judge Beryl Howell granted the House Judiciary Committee (HJC) request for legal authority to receive 6e grand jury material underlying the Mueller report.
Additionally, and most importantly, within the Howell decision she officially recognized the HJC effort was predicated on a constitutional impeachment process.  In essence Howell’s opinion granted the HJC with “judicial enforcement authority.”
The DOJ moved to appeal the decision and requested a “stay” pending appeal.  Judge Howell rejected the DOJ “stay” motion.
The DOJ then appealed to the DC Court of Appeals.  A panel of three judges issued an “administrative stay”, blocking enforcement of the Howell ruling while the appeal was reviewed.  The DC Appellate Court heard arguments within the appeal.

Right now; and considering the House voted on a ‘resolution’ to support Nancy Pelosi’s unilaterally decreed “impeachment inquiry”; and due to the lack of structural specifics within the constitution surrounding the impeachment process; I would put the odds at 50/50 the House Judiciary Committee could win this case based on lower court rulings.
It is critical that AG Bill Barr sends his best constitutional lawyers to defend the interests of the executive branch.  The DOJ has a solid constitutional argument to make; and if they end up losing the decision the verbal arguments will be a key factor in whether the Supreme Court would take up the issue (after en banc appeal exhausted).
The HJC objective is simple.  They seek judicial enforcement authority for their subpoenas so their targets cannot legally refuse to give testimony; and by extension the constitutional premise of the House process is affirmed.
The premise for both fronts: (1) document subpoena 6e material, and (2) testimony from White House Counsel Don McGahn, is predicated on penetrating a constitutional firewall that exists within the separation of powers.
Under existing SCOTUS precedent, the White House can be compelled to deliver Executive Branch documents and testimony so long as an official legislative branch impeachment process is underway.
Judge Beryl Howell was the first person in the judicial branch to recognize and accept the HJC position that such an official impeachment process was ongoing.  At the heart of this appeal is that recognition.
If the DOJ can successfully argue the House has not followed the traditional and constitutional process that authorizes impeachment investigation; and allows the HJC to penetrate the separation of power firewall; it will be a major blow to the Lawfare scheme.
A ruling in favor of the DOJ would invalidate the narrative of the House.
A ruling in favor of the DOJ would also allow the Senate to dismiss any results from Schiff and Nadler’s investigation, because their process would not be predicated on constitutional provisions for impeachment.
In short, this is a pretty important ruling.