New iPhone Xmax. Itunes says will replace media from another library on Macbook.

New Iphone X Max. Want to transfer music from iTunes. Have only had one iTunes Library and have only loaded music from this one Macbook.

When I try to manually transfer music, ITunes message says: "Syncing iPhone with this iTunes Library will replace media from another iTunes Library on Macbook Pro."

Does this mean the iTunes library on my Macbook will be replaced with what is on my iPhone or will iTunes on my iPhone be replaced with what is on my Macbook. There are only a few songs on my iPhone and they are in my Macbook Library, so would be replaced, if replace is from Macbook to iPhone., but ALL would be lost if replace is from iPhone to Macbook.

This is so confusing.... Help!