Lungo is italian for 'long' and does in this context describe the coffee beverage made when using an espresso machine and making an espresso (single or double dose or shot) and in this case letting the amount of water running through the coffee be larger and taking twice the time than normal: usually twice as much or more.
The result is a stretched espresso, a lungo.
A 'normal' espresso takes from 18 up to 30 seconds to 'pull', and fills 2.5 to 3 centilitres, while a 'lungo' may take up to a minute to pull, and might fill 10 to 12 centilitres.
In French it is called café allongé, in German Verlängerter.
A caffè lungo must not be mistaken for a caffè américano, which is an espresso with a little hot water in it. A lungo is richer, but more bitter.