Despite the acronym, GILTI won't impact most US taxpayers. Global Intangible Low-Taxed Income is a new tax law provision enacted for years beginning in 2018. It applies to certain shareholders owning stock in foreign corporations. What GILTI basically does, is include in the current income for these shareholders, an amount based upon the current year foreign corporation income and the amount of the corporation's tangible assets.
The tax calculation for GILTI is quite complex and not for the faint of heart. It shouldn't be attempted by non-tax professionals. There are also potential mechanisms for mitigating the tax depending on the type of ownership you have.
If you have ownership in foreign stock and haven't already made plans for dealing with GILTI going forward, now is a really good time to visit the issue and take proactive measures. If you own foreign stock and haven't already disclosed it you may end up with a tax hit that will really make you feel GILTI.