I've noticed that the acceptance rate for my questions has gone down. What is the acceptance rate? How does it affect my reputation, privileges, or badges?
The acceptance rate is calculated on the answers you accepted in your questions, related to the total number of questions you asked.
For example, if you ask 10 questions and accept 9, you'll have a 90% accept rate. Simple, no?
Note that the acceptance rate starts only after the 5th question. If you ask 4 questions, the acceptance rate is not shown.
Lastly, the acceptance rate does not affect reputation (well, apart from the fact that when you accept an answer you get +2 reputation) or badges or privileges. Privileges are given according to reputation, and badges encourage good behavior.