Why can't I sync songs onto my iPhone?

So I recently just got an iPhone 4, and instantly I have tried putting music onto it. But for some reason I try to sync my phone with my iTunes (where every song on there I imported from my legitimately purchased CDs) and it says all of the items (the songs) can't be synched. Does anybody know why?

Plug your phone into your computer then open up iTunes, drag and drop your songs into the iTunes library then sync it to your phone. Don't move or delete the music on your computer or next time you sync your phone you will loose the music as iTunes library is only a link to the music. Apple devices don't allow the actual song just a link to it on their devices.

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