If you have a fuel tank capacity of 13.8L what does it mean?

It means you might not have heard of the metric system and if you own a Japanese bike (this is posted in the Yamaha moto section), you *need* to know the metric system even for basic maintenance. L is the abbreviation for Liters. The unit Liters is another way to measure capacity like gallons.

If you type into g00gle "13.9L in gallons" and it will do the conversion for you.

you get: 13.9 L = 3.67199153 US gallons