In short it means the pros aka The Market Makers control the market. What that means is they have the ability to see all of the price action which allows them to manipulate the market. When prices go up or down, it's them deciding whether they go up or down or at least throttling the rate to maximizes their earnings. They are both buying and selling at all times. What makes Robinhood such a joke is that they are a middleman between you and a middleman. So instead of buying your drugs from a dealer, you're buying from the pusher on the corner who gets free drugs for every new customer.