It is an abbreviation of "for your information". It is often used for adding info about something that the person whats to say. For instance, you could say "FYI, I'm not going to do that" meaning, just letting you know, I'm not going to do that.

I see. You can use it even at work. It's good to know. Thank you very much for your nice comment, Zhini.

It can be used with a playful tone like Ellen already suggested. However, it is not always that complicated. In most cases, it is used in a straightforward way, which is widely seen in email exchanges at workplace. For example, in my work (college professor), when I forward some information on school events or scholarship application to my students or colleagues, I will just put “FYI” before the forwarded message. When you have nothing to say about the forwarded message other than just sending it out, it is very convenient to use “FYI”. It is also very common to include it in the email subject. It is true that humor is the most difficult/risky thing to learn in a foreign language. But don’t think FYI is very hard to use or non-native speaker should not use it often : )

Thank you, Ellen. >>. It is also pretty commonly used in a sarcastic or negative manner. << I see. It would be difficult for me to use it.

Hi Hanji, It is short for: For Your Information. It is often used to provide background information that might be helpful. Say a friend invites you to a party knowing that you really don't love loud parties but he adds the FYI that the really cute girl you want to get to know is going to be there. It is also pretty commonly used in a sarcastic or negative manner. So, in the party scenario the "not your friend" who hears why you decided to go to the party might say to you: "FYI dude, she will be MY next girlfriend, not yours." Hope this helps. Ellen