Rank 5 means you've played about 25-30~ games (assuming you won most of them).
Using Bard as an example:
http://vgy.me/nnWQCy.png
That puts you in nearly in the top 25%~ for players who have played Bard.
The top 25%~ of players who have played Bard, on average, win about 10%~ more games as Bard than players with only 0-10~ games played as Bard (increasing from 44% to 54%)
Yes. By playing a champion 20+ games it means you're better than most players at that champion.
[I don't mean this to brag, but proving a point about dedication to a single champion.]
I have a Smurf that has only played Vayne for about 300~ games. It's safe to say I'm a better Vayne than most players and a better ADC than most players (seeing as I got to Plat mostly via ADC and Plat is better than most players, while still being terrible at this game)
http://vgy.me/xensqu.png