What does fluency actually mean?
I was looking at my sister's account and I noticed she was a lot less fluent than me, even though she is way ahead of me in the activities and Level, and I am 45% fluent and she is only 40%. I considered the fact of me only doing skills when all my other skills are golden, because she does not strengthen her skills very often... But it didn't really make any sense. Thanks for the help!
There's the dictionary definition, which has no connection with Duolingo's usage of the term. Frankly, it doesn't mean anything here. Ignore it.
Duolingo's Fluency score is based on what you have done using Duolingo. There is an explanation in the Duolingo Help Center here:
"Duolingo estimates your fluency based on your progress through the skill tree and the accuracy of your answers. Because learning a language requires repetition, we also take into account how regularly you practice." - according to Duolingo
It means the ability to speak or write a foreign language easily and accurately.