I can still recall when I was a green photog and first heard this term.  Actually, at the time it was told to me as TFP.  Had no idea what the model was talking about.  So it was immediately clear to her that I was a rookie.  I was okay with that though. . We all have to learn somehow.  I would quickly find out that it meant "Trade For Print".  However, in an a time where so many folks are digital, it's been shortened more recently to TF for simply "Trade For".

TF is a situation when the photographer doesn't charge the model and the model doesn't charge the photographer.  They are making a trade for each other services.  The reason being usually because they are both mutually benefiting one another's portfolio by doing the shoot. 

What some folks fail to realize is it is really only beneficial if it is a mutual trade.  If the model is just starting out, doesn't know what he/she is doing with their look, posing, etc. then it won't really help out the photoog's port a great deal.  Same thing goes the other way around.  Then it's not really a trade.  It's really one party giving the other a handout.  And not to say that doesn't happen because it does.

The whole reason I do TF shoots is for the networking opportunities.  It will often lead to connecting with other clients.  Or sometimes even other opportunities.  Like my most recent TF shoot which now landed me an all expenses paid job for an east coast shoot.  Just more than a little excited about that.  So although TF shoots may not sometimes be cost effective, the ends can justify the means.