1. Buyer A wants to buy 50 shares of Acme Corp.

2. The order hits Exchange A first, and Exchange A has 25 shares of Acme Corp. at $20.00 (currently the best price).

3. The order is partially filled, and then on to Exchange 2 to try to get the other 25 shares.

4. Exchange 2 also has 25 shares of Acme Corp. at $20.00 at the time the order hits Exchange 1.

5. An HFT bot saw the order hit Exchange 1, and, in the time it takes for the order to move on to Exchange 2, is able (due to having much faster access) to buy those 25 shares at Exchange 2 before the order gets there, then offer them to the buyer at $20.01.

The fact that the HFT bot was able to act in Exchange 2 with advance knowledge of an order that would arrive at but had not yet arrived at Exchange 2 is what people complain about. And, more generally, people complain that this provides no benefit to buyers or sellers; the buyer was going to buy those 25 shares, and the seller was going to sell those 25 shares, no matter what. So no additional liquidity or market-making went on, and the buyer's order likely is not filled any more quickly than it would otherwise have been.

(and several articles complain that this practice actually leads to loss of liquidity, in that orders go unfilled because HFT bots racing ahead from exchange to exchange have reduced the previously-available shares)

The workaround that's been presented is to have the actual initial order take circuitous, deliberately time-delaying routes to the exchanges in order to have it hit multiple exchanges within a short enough time that an HFT bot can't inject itself as an artifical middleman.