Imagine two investors sitting at a restaurant table discussing trades they are about to make. The trades they are making will be significant, in the sense that their trades will then impact the value of the stocks they're trading. Meanwhile, a waiter at the restaurant makes a habit of eavesdropping on the conversations of these investors. When he gets the information, he runs to the phone and effects his own, smaller trade.
It's not that the waiter happened to overhear something. The waiter makes it his business to "overhear." The waiter adds no real value. He's a parasite on the people who do add value. The HFT traders are likewise.
What they do is documented in Michael Lewis's Flashboys.
You might want to read this first.
In this book it talks about how in the early 2000s fiber cable was the fastest thing and most expensive thing anyone could possibly get so that's what the stock market used but today they don't use Fiber anymore. They use various types of microwave Wireless way faster than fiber, why? Light bouncing around inside of a cable travels a much further distance in a straight line than light transmitted between two points with line of sight.
This is very similar to "front running" , but because a) this happens privately and in small quantities, and b) it is not necessarily privileged information, it's considered legal by the SEC. But it is highly questionable for sure.
"Flash Boys", by Michael Lewis  is a great read, and goes into detail on the world of high frequency traders, and how they operate.