It is literally when packets are lost in transit from point A to point B. If you are on WI-fi, this could be happening quite frequently. There can be disturbances from other frequencies, such as microwaves being turned on, or just actual physical objects moving and blocking the signals.
If you aren't on wifi, then I don't know what to tell you, but you can press f5 or ~ to open the console, and type 'stat net' for the network statistics to open, and you can look at in/out loss there. The same command removes the stats.