It has to do with how people/companies called "market makers" make money. Let's say you want to buy a share of stock. Generally you don't end up buying it from another random person who happens to want to sell a share of stock at the exact same time. There might not even be another random person that wants to sell right now.
Instead you are likely buying from a market maker. For pretty much any stock there are many market makers who are offering to both buy stock and sell it at the same time. The trick is they'll buy for, say, a penny less than they are willing to sell for. So if they can keep their buys/sells even they make a penny for every share they move. They're middle men.
Make sense so far?
OK, but there is a problem. Say that REALLY BIG HEDGE FUND has proprietary knowledge that some company is probably going to go up in value soon. So they go out and buy a lot of stock from a market maker. And then the stock goes up. Uh oh. The market maker sold a bunch of stock at PRICE and now instead of buying an equal amount at (PRICE - 1 penny) and making money the market maker has to buy an equal amount at (PRICE + the_stock_went_up_amt) and loses money.
Basically market makers would prefer not to trade with sophisticated investors making big trades with proprietary knowledge. When that happens and they lose it's called getting picked off. They want to trade with people like you and me who aren't trading because of any special knowledge but because we're just putting our regular $1,000 in our 401k for the month.
Retail brokerages like Robinhood are a good source of these types of trades.
(As with a lot of topics, there is a lot of nuance and detail underneath this relatively short description, but this is the gist of it.)