Basically, if you want to buy AAPL at $100 and as your broker, I take that info and share it with someone else, such as an HFT, they will quickly (milliseconds/nanoseconds) buy AAPL and sell you at a higher price. So, while you expected to pay $100, you ended up paying $100.10.
Now that doesn't seem a lot to you but times the difference (10 cents) by volume and number of Robinhood customers placing orders and it can add up to be a lot.
Now that doesn't seem a lot to you but times the difference (10 cents) by volume and number of Robinhood customers placing orders and it can add up to be a lot.