They send in tens of thousands of requests a second, many of which they have no interest in ever being forfilled, in the hope of partly fooling other people, who are sending around similar numbers of requests.
Stock exchanges have turned into a high-frequency war-ground, which fortunately doesn't appear to spill out and effect the rest of us too often, at least as far as I understand.
In any case, to the 'end user' (a regular stock investor like you or me) the continual jockeying for position among HFTs doesn't matter - all we notice is that we pay smaller spreads and get our orders filled more quickly.
Sometimes the trading games spill out into the 'real world' with unpleasant consequences, but those instances appear (so far) to be rare. I think there needs to be tight regulation on HFTs to limit behaviour like quote stuffing, spoofing and other forms of gaming, but I don't agree with the argument that they are a net bad thing.
Besides, most retail orders go through brokers, not directly to the exchange, and the brokers can provide their own layer of algorithmic sophistication to help the clients get a better price than if they were to place the orders on the exchange on their own directly. This can be done by internally matching orders on both sides, as well as placing orders at strategic times and prices, also using algorithms.
So, this isn't really about "poor retail investor" against "evil predatory high-frequency trader".
That's called gaming, and is illegal.