For a discussion on anki's definition of "mature" cards I recommend reading their manual (http://ankisrs.net/docs/manual.html) and searching for "mature".
You can also ask on their forums.
I haven't researched this in detail. It certainly doesn't mean that you know it 100%. But I'm guessing that there is a relationship between card interval and liklihood of you getting it right. Like, you're more likely to forget a card with a short interval than a card with a long interval. That relationship is probably really clearest for cards with a short interal (e.g. words you just learned), and then it probably peters out. I'm not sure how they picked 21 days as the cutoff.
For me, I found counting mature cards more useful than counting total cards. Because I can't really pad the number of mature cards. For example, if I counted mature + learning cards I could just throw in a bunch of words at the end of the month and be like "hey look at me! I learned so many words this month!"
You can also ask on their forums.
I haven't researched this in detail. It certainly doesn't mean that you know it 100%. But I'm guessing that there is a relationship between card interval and liklihood of you getting it right. Like, you're more likely to forget a card with a short interval than a card with a long interval. That relationship is probably really clearest for cards with a short interal (e.g. words you just learned), and then it probably peters out. I'm not sure how they picked 21 days as the cutoff.
For me, I found counting mature cards more useful than counting total cards. Because I can't really pad the number of mature cards. For example, if I counted mature + learning cards I could just throw in a bunch of words at the end of the month and be like "hey look at me! I learned so many words this month!"