When you /t it means to send that person a tell.  If you wanted to send me a tell then you would type /tBlackbeard the Enter (then type your message) in the chat window.  When you are done with your message you hit Enter again and that person will see whatever you typed.  Since there are so many chat windows that one person could be chatting in, a /t shows up to that person in all of his/her chat windows.

That is the best way I can explain it.  I hope it helps.