This is short for face to face
. It is used frequently in email, on irc, etc. It refers to having a conversation with someone, in the same room - talking.
We frequently used term at work in email, to let the person know that if we need to talk to them, that the telephone isn't good enough.
Example: "Get a hold of me later - f2f", or "He is so concise in email, but f2f he rambles".