Say you have a bank account with $100 in it. At 100% reserve requirements, the bank must have that $100 actually in its possession at all times. This is very safe, because you are guaranteed to be able to withdraw your money in the event of a bank run. However, an economist might believe this is suboptimal because the $100 is just sitting there doing nothing rather than "circulating in the economy". Thus the concept of the reserve ratio is born: banks don't keep that $100 around, but instead lend a percentage of it out or do whatever weird trades they want to do to make money. This is called fractional-reserve banking.
Before this change, reserve requirements in the US were at 10% which means the bank could lend out $90. Now in general this is problematic because if everyone were to try to withdraw their money at once, the bank could not produce it. Thus we have the FDIC, which is a government program that insures bank account deposits up to $250k so people don't lose all their money if a bank run happens. A cynical way of looking at this is that taxpayers are on the hook for the bank's bad behavior if they go bust, and the lower the reserve ratio the more likely the bank is to overdo it.
Things also get really weird when other banks are in the picture. If you deposit $100 at bank A which has 10% reserve ratio, it will lend out $90 - so total money in the system is now $190. That $90 might end up deposited at bank B, which also has a 10% reserve ratio, so it lends out $81 making the total amount of money in the system $190 + $81 = $271. This process continues until it approaches the value of principal * (1 / reserve ratio), so in our case $100 * (1 / 0.1) = $1000 [0]. At 0% this means technically infinite money can be lent/created, although banks will presumably keep some percentage as reserves due to internal risk requirements (interesting assumption, I know).
Reserve requirement ratios are viewed as one of the levers the government can use to help steer the economy. Reducing the reserve requirement ratio has the effect of increasing the overall money supply, which has some nonlinear effect on the economy that the government wants to happen. This move to 0%, in addition to various other recent moves by the federal reserve, are all geared toward flooding the economy with cheap easy money to prop it up. They will all fail.
[0] https://www.economicshelp.org/blog/67/money/money-multiplier...