Shameless copy/paste from a comment on reddit that summed it up well imo: "Answer: at an ELI5 level, Silicon Valley Bank (SVB) is a bank that focuses on providing services to startups and entrepreneurs. Many companies use it to hold funds that they receive from venture capitalists. In 2021, the market was soaring and startups were getting tons of money. They put this money in SVB, which went from holding $61.76bn at the end of 2019 to $189.20bn at the end of 2021. Banks normally make money by loaning out a portion of the money they hold, but SVB was getting so much money that they couldn't loan out fast enough. So instead, they bought a bunch of long term investments, the majority of which will mature in 10 years. This would be okay except that when the fed started raising interest rates last year, the value of these long term assets fell hard. Simultaneously, tech and startups also started to struggle with the rate hikes (see: all the big layoffs) and pull from their deposits more quickly. Yesterday SVB announced a fire sale: they were going to sell pretty much everything they could sell in order to raise cash and balance out all those long term assets with more short term assets. They sold over 21 billion worth of investments and are trying to raise 3 billion more. Investors and Venture Capitalists were shocked and concerned about why they had to do this and why they had to do it now. Some VCs told their startups to pull their money out of SVB or to keep no more than 250k in the bank. This has raised concerns of starting a run on the bank. SVB is theoretically fine right now, but if all of these startups try to pull their money out they won't be."

So basically, if all these startups try to pull their money from the bank, they won't get it because the bank doesn't have the money to give?