If you're a broker/dealer, you spend a lot of your time facilitating other people's trades. By "facilitating" here, we mean that you don't only put their trade on the market for them, you often trade with them "on risk" by taking the other side of their trades and then unwinding them. So if you want to sell 1000 tesla I might (as a broker/dealer) just buy them off you and look to sell them myself later either at market or directly without touching the market as part of executing someone else's trade later. This is more efficient for everyone as it means you don't pay the exchange fees for those trades you can "cross off" against a colleague or another client and therefore can offer a slightly better price to the end client. It also allows traders to manage the market impact of big orders more effectively, allowing large trades to be completed without moving the market as much.

If you're large enough, your client base represents the market generally. That means your client base by definition doesn't outperform the market (ie has zero alpha). So that means that facilitating their trading earns you the commission and the trades you have to unwind have net zero alpha. This is not entirely true because it ignores some important effects around how commissions work etc (which end up meaning that broker/dealers are structurally long the market in general) but is not false enough to matter for the purposes of this discussion.

Imagine you had one client who knew the future (ie every trade they made would make them money). By definition taking the other side of that trade would therefore lose you money. Their orderflow would be "toxic" - by trading with them you would always lose money.

When someone says that orderflow is "informed" what they mean (usually) is that the people making the trades have more information than the rest of the market and therefore will trade when beneficial to them which is likely to be net/net not beneficial to you (if as a broker you're on the other side of the trade).

Now, whether or not robinhood order flow is on the whole informed or toxic is another question. Personally I would be surprised if that turns out to be true but I could be wrong.