I can address what it means to "pick off" all those trades... As quantycuenta & littleadv have said, it is absolutely true that professionals "prey" on less-sophisticated market participants. They aren't in the market for charity's sake.
If you're not familiar with the definition of the word "arbitrage", look it up.
One possible strategy that can be employed with HFT machinery in order to arbitrage successfully in the stock market is to 'intercept' orders that are placed on various exchanges. In order to do this, an HFT organization watches all the transactions at once to find opportunities to buy low and sell high.
A good explanation of it is described here in this NY Times article; I'll paraphrase what that article lays out.
Stocks are traded through multiple exchanges
The first key point to understand is that stocks listed on one exchange (i.e. the NYSE) can be sold on multiple exchanges. That's where the actual "I would like to sell 100 shares of Ford stock" is matched with "I would like to buy 100 shares of Ford stock." There are multiple clearinghouses on the various exchanges.
Your order gets presented to one exchange at a Time An ideal market maker would like to look at the order books for a given stock, say Ford, and see that in exchange A there's a sell order for 100 shares of F at $15.85, and in exchange B there's a buy order for 100 shares of F at $15.90.
Arbitrage Market maker buys from A, sells in B, and pockets $0.05 * 100... $5. It's not much, but it was relatively risk free. Also, scale this up to the scale of the US' multiple stock exchanges, and there are lots of opportunities to make $5 every second.
Computers are (of course) faster than people To tie it in completely with your question about 'picking off trades', HFT rigs can be set up and programmed to go faster than an average retail investor's order.
Let's say you execute the trade to buy 100 shares @ $15.85 as a retail investor. The HFT rigs see your order starting to make the rounds of the different exchanges that your brokerage works through, and go out in front in a matter of milliseconds, finding the orders that are less than $15.85 and less than or equal to 100 shares. They execute a transaction, buy them up, sell to you, and pocket the difference.
You have been "picked off".
It's admittedly not the only way to use HFT equipment to make money, but it's definitely one way to do it.