Definition
Ping is a network checking tool that measures how long it takes for a signal sent from your computer to reach another computer or server and come back. It tells you whether your internet connection works and how fast it responds, in milliseconds (ms, one thousandth of a second).
Bounce a Ping-Pong Ball to See if There's a Wall
When you shout from an open mountaintop, an echo comes back after a moment. If it returns quickly, the mountain across from you is close. If nothing comes back, there's no wall to bounce off, or it's too far away.
Computers check on each other's presence and distance in the same way. Your computer throws a tiny data signal at the other server that says, "Are you there?" The other side sends back a reply: "I'm here!" The technique of timing the round trip of the signal over this whole exchange is called ping.
The person who created it said he named it after the sound a submarine's sonar (sound-wave detector) makes when it sends out a pulse to find objects. If no signal comes back, the other computer may be turned off or the cable may be cut. It's also possible that a firewall is blocking ping replies on purpose for security. So a failed ping doesn't always mean something is broken.
A Bit More Precisely: ICMP and Round-Trip Time
To be more precise, ping uses a set of network communication rules called the Internet Control Message Protocol (ICMP). The signal your computer sends when it pings is called an "Echo Request," and the signal the other side sends back is called an "Echo Reply." The word for a repeated sound shows up right in the name of the technology.
The total time from when the signal leaves to when it returns is called the round-trip time (RTT). A number like "ping: 15ms" on your screen is this round-trip time. 15ms means the signal took 0.015 seconds to go and come back.
If you send 4 signals and only 3 come back, that's 25% packet loss (lost data). With ping, you can tell not only whether you're connected, but also how stable the network is, at a glance.
Download Speed and Ping Are Different Things
Many people think a fast connection, like gigabit internet, will always bring their ping close to 0. But download speed (bandwidth), which is how fast you can receive big files, and response speed (ping) are completely different ideas.
If bandwidth is the number of lanes on a road, ping is the speed limit and the distance you have to travel. Even with 100 lanes, if the physical distance from Seoul to New York is long, the signal still takes time to arrive. No matter how expensive your internet is, when you talk to an overseas server your ping goes up because of the limit of physical distance.
In real-time competitive games and online shooters especially, ping matters far more than bandwidth. A low ping number means that the attack you made by clicking your mouse reaches the server as fast as lightning.
Common misconceptions
If your download speed is a fast 1Gbps, your ping is always close to 0.
Download speed is how much data you can carry at once (the number of lanes), while ping is the delay in a signal's trip back and forth. Delay depends on distance, route, and the processing time of equipment along the way, so even on a very fast connection, ping can't get close to 0 if the server is on the other side of the planet.
Where you meet it
It's the response delay shown in the corner of an online game screen, like "25ms."
When your internet is down, you use it to send a signal to your router or a portal site's server to check whether something is broken.
In one sentence
It's a tool that sends a signal to another server and measures how long it takes to come back, to check the state of a connection and its response speed.