Ping (latency is the technically more correct term) means the time it takes for a small data set to be transmitted from your device to a server on the Internet and back to your device again. The ping time is measured in milliseconds (ms). The ping time is a very important indicator for online games, but for quite normal Internet surfing too the ping time has a considerable effect on the access delay time. The delay is critically influenced both by your Internet access technology and by its utilisation level. Note that ping refers to two-way latency (aka round-trip delay), a value relevant for Internet usage. Sometimes it is referred to "one-way latencies" which account only for the access part, but not for the transmission in the whole network.
If you are accessing very far remote servers, the length of the transmission route also has a marked effect on the ping time (e.g. about 280 ms for Japan). Fast Internet connections have a ping time of less than 2 ms (optical fibre), slow Internet connections may have ping times of over 50 ms (UMTS access); typically ping times range from 5 to 30 ms.