Every nutrition label is required to have grams of carbs per serving on it. This should be on the left right after "total carbohydrates."
The percentage is based on an assumed average diet consisting of 2,000 calories and whatever kind of macros the government decided. Pretty much everyone...
You should be able to click on that icon to get some more information. The manual calls that a "travel" icon. So something about the app thinks you were traveling for that reading. It might have something to do with the location you have configured in the app.