So must of us are familiar with federal income tax. As an example if you have a job, the amount of money that you earn (after considering deductions/credits) is taxed in different "brackets". If you are taxed on 80,000 dollars, ~10000 of it would be taxed at a 10% rate, and 30K of it would be taxed at a 12% rate, and the remaining 40K would be taxed at 22%. If you earned over ~500,000 dollars, that money gets taxed in the highest bracket, which is 37%.
Another type of tax is a capital gains tax. So lets say you bought some stocks, and held on to them for longer than a year. If you initially paid $10 for that stock, and it was worth $1000 dollars when you sold it 2 years later, you would owe a capital gains tax on the $990 dollars that you made when you sold the stock. The capital gains rate is 15%.
So if you're very wealthy, and your stocks are doing well, then when you sell them, you're only paying capital gains. So it's possible to make a lot of money, but to be taxed at this lower rate.
If you're very, very wealthy (Let's say a tech CEO/founder) then you really don't need a huge salary, and thus you pay very little in income taxes. Instead, you can live off of selling stock. Furthermore, it's possible to use your stock as collateral and borrow money, which you don't have to pay taxes on.
For the ultrawealthy that this would target, most of their assets are in stock. If their stock doubles, their net worth doubles. Likewise if their stock value halved, their net worth would half. When you hear that Bezos or Musk made x amount during the pandemic, this is primarily because their stocks have done well.
Under the current system, they aren't taxed just because their stock went up, they would only get taxed if they sold the stock, and then they would pay capital gains.
The proposed system would have a metric to make them pay capital gains even if they don't sell their stock.
The positive is that it generates revenue and is a small measure towards helping with wealth inequality.
The negative is that it's a tax on unrealized gains: you don't actually "have" the money until you sell the stock. If their stock goes up by a certain amount, they may be forced to sell stock that they otherwise wouldn't have.
Hope that helps