I'll give it a shot. When you place an order for 1000 shares of AAPL for example your order is sent to a bunch of exchanges and you are supposed to get your shares from the 1000 cheapest shares available to be purchased. If your order gets to exchange A where a HFT is operating they might sell you 100 shares at $131.99, but then the know there is someone buying AAPL. they send orders on faster networks than you to buy up all the shares at other exchanges for 131.99, then list them for sale for 132.00. Your order arrives, and depending on order type you'll probably buy those up that milliseconds earlier would have been a penny cheaper.

For you this is only $9.00 more than before, but doing this all the time is where HFT makes money, without really offering any benefit to other market participants.