HFT, on its own, is exactly what the name suggests- extremely fast, high frequency trades. Instead of placing a trade per second, High Frequency Trading firms have reached a point where they can place multiple trades within nanoseconds. Not milliseconds (1/1000th of a second) or microseconds (1 millionth of a second), but nanoseconds- that is, 1 billionth of a second.
Latency arbitrage is the practice of buying or selling an trading instrument slightly ahead of other market participants, by taking advantage of small delays in price dissemination. So Measuring the Latency between Exchanges makes sense when comes to Inter Commodities Hedging or Inter-Country Latency arbitrage.