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.
OPは自分が誰であるかを書き込み、他のユーザーは質問したいことを何でも自由に尋ねることができる。
「the OP writes who OPs are and other users are invited to ask OPs any questions other users have.」