PLEASE HELP What does it mean all existing content on ipod will be replaced with content from itunes libray?

I am so mad... there's videos that are on my ipod nano and there are on the same format because I see them in itunes but the problem is that it won't sync!!Not even music will let me anymore :/ Idk what to do I am try to check off "put manually manage music and videos" to see if that will work... or restore by its original settings but that will erase everything...oh please please help meeee! I tried so hard to be able to manage drag the videos in itunes but it won't sync... Any advise?????