is not a rank itself, yeha, you need to reach legends first, but in order to reach 11 stars yoou need a really good winrate to increase your mmr. what would be called a high legend player
No, you don't need to reach legend, just to be within an certain top % of players based on MMR. I actually did just that a season I had a very good winrate but didn't play quite enough games to reach legend.
It is Match-Making-Rating, a number which should show your skill-level pretty accurately given some time. It is used by the matchmaker to give you opponents at a similar level. On the Hearthstone ladder, this number is unfortunately hidden, but in Battlegrounds it is shown.