When I said "where the market is going to be" I meant that from one second to the next, you knew it wouldn't move that far.

In the past, when you called up your broker to buy a stock, he could give you a quote (say 12.35) and you tell him to buy, you know that the price will be the quoted amount. (Market in finance lingo in this case means current price of the stock)

If you need to sell a large batch of stock, you know the broker can sell it to multiple people and get you the best price.

With HFT, trades are moving so much faster and you can get a quote from the exchange (someone want to buy 500 shares @ 12.35) and by the time your trade gets sent from your computer to the exchange, that offer is now 400 shares at $12.50. The market has moved in milliseconds. In other words, you cannot trust the market (current price) because you don't know where it will be in the milliseconds from viewing the quote to entering the trade.

In Flash Boys, they talked about ghosts in the machines. This is what they were referring to. When they entered their trades, it's as if the market knew beforehand and moved against you.

(For finance savvy people, apologies, I've simplified it in lieu of understanding)