This is just my non-expert understanding: They give you the share immediately, but the actual trade takes two days to complete, and might not complete at the pr

This is just my non-expert understanding: They give you the share immediately, but the actual trade takes two days to complete, and might not complete at the price you paid. They're in affect giving you a loan for that two-day period, and the DTCC gives them a loan. The DTCC wants to make sure robinhood won't default on their loan, so they require robinhood to post a certain amount of collateral. Usually this isn't such a big deal, because stocks don't vary much day-to-day so the collateral requirements aren't very high, but for extremely volatile stocks like GME they might need to have $100 of collateral for every $100 of buys they try to execute. They couldn't afford that so they stopped accepting buy orders. TD Ameritrade, another broker, stopped accepting buy orders for GME a day before Robinhood did for the same reason. (Robinhood has suggested that trade should settle in minutes or seconds instead of days, which would obviously be better, but that won't happen any time soon)