It is often used to give more information and clarify, so to make the meaning clear. An extended version of it could be : "by which I mean" For example: He is slow, as in he's not as fast as his brother. I like tea, as in I drink it every day. Democracy, as in the running of the country in which everyone has a vote. A: Do you like chocolate? B: Like...as in? A: As in love. B: Yeah, sure, I love chocolate!