Why are my songs greyed out on my iPhone 4s?

I have an iPhone 4s and I had it plugged into my computer I have songs I don't listen to so I deleted them all. I went on my phone and All THE SONGS were on my phone but greyed out! I tried to delete them but it didn't work! I don't know what to do and i'm not really liking iTunes anymore!

Resync your iPhone 4S with iTunes.

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