Running line is the section of line after the rear taper and extends all the way to the end of the line. This section of line is sometimes referred to shooting line as well.
The running line is usually the same diameter all the way through and is smaller in diameter then the rest of the line. This smaller diameter allows the line to glide freely through all the guides when shooting line to your target.
It is very difficult to transfer energy efficiently from a smaller diameter line to a larger diameter line. When you're false casting and carrying a large amount of line in the air, usually your running line is in the rod tip. This calls for a substantial increase of power to continue supporting your loop. If the loop falls apart your cast you will lose distance and accuracy.
I will break down this topic as it pertains to casting in future posts so stay tuned.