New iPhone 6 - "erase and sync" to iTunes library to add non-iTunes music

I'm handy with technology, but not a tech head so bear with me.

My iPhone 5 16GB died so I bought a iPhone 6 64GB. Set it up and then "restored" my old iPhone 5 info from my iTunes backup onto it. So far, so good.

I want to manually add music from the iTunes library onto the new phone. The Music app on my new phone shows I can download from the Cloud any songs onto to it that I purchased thru iTunes. Fine. But I want to move over some of the many songs I added to iTunes over the years from CDs, other online services, etc.

When I click "Manually manage music and video" and try to sync music I get the infamous "The iPhone is synced with another library. Do you want to erase this iPhone and sync to this library?"

This sounds like what I should do without worry as I have the phone recently backed up to the Cloud AND my iTunes, and have done little with the new phone since buying it. I've put all the iPhone photos on a hard drives, too. So if i "erase and sync," will this just repopulate my phone with all the previous info backed up on iTunes (photos, contacts, calendar, apps, etc) PLUS the music I want to manually add from the iTunes library?

I know this may sound elementary to some of you, but honestly - I can't find any clear explanations online of what will be deleted and what will be added with the "erase and sync" in this situation.

Thanks, all.