It’s hard to summarize but basically they’re trying to decide if part of the 2017 Tax Cuts and Jobs Act is constitutional or not. It’s the part requiring taxpayers to pick up income for their share of income earned internationally that hasn’t been repatriated yet, essentially unrealized gains. This has broad implications on whether a wealth tax on unrealized capital gains (looking at you, Musk and Bezos et al) would be constitutional under the 16th Amendment which only authorizes an income tax.
I haven’t done a deep dive on that case though so I could be wrong, comments welcome.