I have seen the expression "fl. oz" accompanied by a number in a lot of labels of beers. For instance, I have seen "9.4 fl. oz", "12 fl. oz", "11.2 fl. oz"... What do these numbers mean?
It sounds like you live in a place that uses the metric system. A fluid ounce (fl. oz.) is a measure of volume in U.S. customary units: 1 fluid ounce = 29.5735 milliliters. A Fluid ounce in British Imperial Units is slightly smaller: 1 fluid ounce = 28.413 ml. However, you are unlikely to find any modern labeling that uses British Imperial units since the U.K. has fully adopted the metric system, so in all likelihood the labels you are seeing refer to U.S. customary units.
Fluid Ounces, even though it sounds like a measure of weight, is actually a measure of volume. Specifically, 1 fluid ounce is the volume of 1 ounce (by weight) of pure water. Similarly, in the metric system, 1 ml (or cubic centimeter) has 1 gram of mass, so often times (even in the US) you'll see ml listed as the volume as well.