subreddit is an online community
r/IAmA r/IAmA is a subreddit for question-and-answer interactive interviews termed "AMA" (short for "Ask Me Anything").
"OP," meaning "original poster." And during an "AMA" — or "ask me anything" — the OP writes who they are and other users are invited to ask them any questions they have.
「the OP writes who OPs are and other users are invited to ask OPｓ any questions other users have.」