Stock Options == the right purchase a share at the current price at a future date, usually while you're employed by the company. Putting people in the bind of "do I spend a bunch of money to buy this thing which is currently illiquid on the off chance that it's worth something some day".

Tax implications abound on the type of options you have (ISO vs NSO) and the spread of your strike price and the current fair market value of your share. We tend to think of options when we think of tech startups but alternatives have become much more common.

RSU == the right to be GIVEN a dollar amount equivalent of shares at a future date. No strike price, which is great. And if you're at a public company, this is a liquid asset now, so while you now owe taxes on this thing you were just given, you can also sell it to pay those taxes. Relatively risk free. If you're a private company however, then RSUs are the right to be given something at a fixed date that has immediate tax implications with no liquidity.

enter "Double Trigger" RSUs == the right to be granted shares when the company goes public (technically "a liquidity event", there is also going private and being bought). Now you've been given something that is valuable, you didn't have to pay a strike price, so aren't risking your outlay, and you can sell it.

That is what Stripe was issuing, and the good news is their current and former employees didn't have to pay taxes or a strike price or walk away from their vested option, but that magic was achieved because they don't technically own those shares. If Stripe never has a liquidity event (or if it goes bankrupt) or if it merely goes too long without that liquidity event then those shares expire having never been "double triggered".

That said I have no idea what Stripe is planning to do about it that will incur such a large tax liability that they need to raise another $4b. A couple of possible approaches but that would be pure speculation.