HELLLPPPP!!!! If I remove all existing music from my ipod and sync to iTunes library i wont be able to get any music back?

So my ipod is still iOS 4 or somthing and I'm was going to update it on my laptop

when I go transfer my puschased iteams on my itunes it says "Are you sure you want to remove all existing music, movies, tv shows and tones from this ipod and sync with this itunes libary?"

I'm so confused what will it do? and what will it mean???

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