Basically, /r/IAmA is a place to interview people, but in a new way. “IAmA” is the traditional way of beginning the description of who you are; “AMA” is the traditional way of ending the description; the acronym means “Ask me anything.” The interviewee begins the process by starting a post, describing who they are and what they do. Then, commenters leave questions and can vote on other questions according to which they would like to see answered. The interviewee then goes through and responds to any questions that he/she would like, and in any way that he/she prefers.
The most popular threads receive thousands of comments and questions.
If you want to read just the questions and answers head over to /r/tabled/. The /r/tabled/ contains questions and answers from subreddits such as /r/IAmA, /r/AskReddit and /r/AskScience in simple question and answer format.
Hopefully this makes it easier for you to check all the answered questions.