Changing iTunes library on my iPhone

So I would like to sync music onto my new (since my old was replaced for damage) phone from my mac but it won't allow me to do so since I first downloaded music with my aunt's macbook with my first iPhone a long time ago. In order to change my phone's iTunes library to the one on my mac, I clicked "sync music" and "Manually manage music and videos" on iTunes but these changes can only be applied by "erasing this iPhone and syncing with this iTunes library".

One of the many things I'm confused about is that I was able to download and sync music onto my phone, before it was replaced, from my mac's iTunes library for a long period of time until I clicked some of the wrong buttons one day and i just wasn't able to download or sync music, I really don't remember what happened or the reason why since it was a while ago. Since then I haven't synced more music obviously, and since getting my new phone, have been living off of Spotify and pandora. But now I'm finally trying to sync my own music.

I'm not sure if this is relevant but another thing I should add is when I synced music on my old phone, it was from the mac profile my family uses and iTunes was logged into my brother's apple ID. I also backed my phone up on that mac profile. I am currently trying to use my own mac profile and iTunes logged into my apple ID, hence the complication with having to erase my phone to use this iTunes since initially syncing from my aunt's mac years ago.

My question is does "erasing this iPhone" mean deleting everything in my phone including apps, photos, contacts, etc.? If so, how can I retrieve all of my content back after erasing to sync with this iTunes library?