This is the time in milliseconds it takes for a request from your computer to reach a server and to return, like the ping of a ship’s sonar. The higher the number, the slower the speed.
Latencies under 100ms are good, under 50ms are great, and ping times over 500ms (half a second!) begin to feel painful.
Unusually high latencies are often a sign of an unreliable connection.
This measurement is particularly important for online gaming and two-way video - but any interactive task can begin to suffer from higher latencies. For general surfing, it will be noticeable as that blank pause when you first request a new website.