why do you get the wanring message when updating?

after reading this i dont understad why, even after backing my ipod up i get this message from itines when i click update.

there are purchased items on the ipod "(my ipods name)" that have not been transfered to your itunes libary. you should transfer theses items to your itunes libary before updating this ipod. are you sure you want to continue"

ive been saying cancel as i dont want to risk losing anything, does anybody know why it says this and/or how i transfer it to my libary.

i have tried quite a bit. the computer is autherised to my account and i have logged in.

also the message doesnt say (my ipods name) but i dont want to say my name!

i would greatly appreciate any help, as my ipod is still on update 4.3.1

(i think)