First question is first, have you watched the DX6i videos that I made a couple years ago? If not, they are a little more boring than my videos today (just started then lol) but they will help you out a lot in learning what all of this is.
DR stands for Dual Rate, meaning you can set one position of the switch for a low rate or movement and the other for high. Rates low and high just means the amount of throw on the control surfaces. More throw, higher rates. Less throw, low rates.
To watch the videos on the DX6i, click on Articles just below the site logo and in the left side of the page click on DX6i in the menu. That will open up more menu's from explaining the Tx to doing a full setup with a 450 size helicopter from start to finish. Watch the videos if you have not done so yet. They will teach you a lot if you are just getting into this tx.