I've prepared income tax professionally for most of the past 35 years. Everyone is aware of the "regular" taxes. But only the big earners have to deal with the minimum tax. It's also called the alternative minimum tax or AMT. If you are able to take advantage of too many tax breaks, the AMT adds them back into your taxable income and subjects them to a separate tax calculation, and you pay the maximum of the two calculated taxes. High earners typically encounter this tax when they have to add back in their itemized deductions as taxable income.
There's a corporate version of this. Biden wants to add back into corporate minimum taxable income foreign deductions, foreign tax breaks.
As a rule, the more complicated your tax situation, the more ability you have to shift around the tax numbers in your favor. This becomes extreme when you're dealing with patents and intellectual property, such as software. There are large gray areas, and you may shift the data to your own tax benefit. This has allowed companies to locate their theoretical sources of revenue to tax haven countries which have the lowest taxes. Biden wants to add much of this back into the corporations' American minimum tax. While legislatively difficult to construct the wording for this, it can be done.
While I am in favor of low taxes, as low as practical, I support Biden's effort to add foreign tax breaks into the corporate minimum tax. Companies shouldn't have an incentive to shift their theoretical income to low tax countries.
Biden also wants to form an international coalition of minimum tax countries. There will always be countries that don't sign onto such a plan. But those that do will have increased clout to coax low tax countries to get with the plan. A minimum tax coalition would decrease the ability of companies to shop for low international taxation.