I'm a little foggy about what the "Target Retention Rate" selector does on the account menu. Would someone please fill me in?

If you took a quiz on everything you've added to Skritter, your retention rate would be your score, with 95% being a good target. To learn faster, lower your retention rate a bit--you'll have much fewer reviews due, but you won't know quite as much of the material at any given time.

Basic version of the theory: when you've just practiced something that you know, your chance of remembering it is 100%. Then it declines gradually as time goes on. By the time the review comes, it'll be 90%. But at any given time, items are on average halfway between reviews, so your average retention will be about halfway, too, around 95%. (The average formula is a bit more complicated).

You can learn more about it at SuperMemo's page:
http://www.supermemo.com/help/fi.htm

If you change your target retention rate, it will take a while for Skritter's scheduling to catch up and adapt to the new rate, as the items you review before the change are still targeted at the old rate, and the intervals themselves have to adapt to your changed target before they'll go down.

@Nick how does the "add word frequency" affect things? I have mine set to "from time to time'

The higher the frequency, the sooner you'll start seeing new words added, in terms of the readiness % of the current item you're practicing. Each option bumps the readiness target by 10%. So if "from time to time" adds such that you usually get to around 90% by the time it's adding a lot of words, then "pretty frequently" would shift that to 100%, "early and often" to 110%, "infrequently" to 80%, "rarely" to 70%, etc.

If you don't often hunger for new words more quickly than Skritter gives them, or aren't overwhelmed by how many words it's adding, then it's probably good to leave it at the default. It's easy to experiment around with, though.

@Nick okay thanks. I'll test it out