Anti-ghosting is a technology used in mechanical keyboards to ensure that simultaneous button presses do not cause errors with the keyboard. Typically used in gaming keyboards, anti-ghosting is usually measured by the number of keys that can be pressed simultaneously before errors occur.
When you attempt to press three or more keys down on most standard models, the keyboard may display one or more errors. In some cases, button presses will not register while some may work but only very slowly. In yet more cases, pressing 3 or more keys at once can lead to the keyboard registering keystrokes that haven’t been pressed. Strictly speaking, it is these phantom button presses that occur when a keyboard is ghosting.
Standard keyboards that only allow for 3 buttons to be pressed at the same time are not ideal for gamers. Consider a first person shooter, you might be moving forward, ducking, and shooting at the same time but because you’ve pressed three buttons at once, you find that you’ve popped a grenade at your own feet. Anti-ghosting, then, prevents phantom grenade throws, or phantom braking in racing games, and so on.
You should check the anti-ghosting capabilities of a gaming keyboard. Some offer anti-ghosting but only on specific keys. For example, the arrow keys or WASD block might all run independently which means that all of these buttons can be pressed at once, and the presses will register properly. However, the rest of the keyboard may use the same column and row layout that leads to ghosting.
If you are looking for a full anti-ghosting gaming keyboard, you should look for one that offers a rating according to the number of keys that can be pressed at once. It is not uncommon to get high end gaming keyboards that allow 20 buttons to be pressed at once, accurately registering every single press. It is possible to find keyboards that would effectively let you hold the whole keyboard down and still translate your presses reliably and accurately.