What does syncing my iPhone mean? - 1

I have a 13 inch Macbook Air and an iPhone 5S with updated software. I want to put a ringtone onto my phone. I already have it set up, but when I want to put it onto my phone, it wants me to "sync" it. It says that it's going to clear the iTunes library on my phone, which has music my Macbook doesn't have. People online say that it's just going to add the songs from my Macbook, and not replace the ones on my phone. Which is right?

To sync means to connect… So basically what you are doing is adding the stuff from your mac book, and the things on your phone will stay.
Can you try to answer my question?

An iPhone can only use one computer at a time. If the phone was set up with a different computer and different iTunes library it will erase the contents of the phone and replace it with the new selections.
"Syncing" means it will put the same stuff selected on the computers iTunes account onto the phone. You can select sync only checked songs.
The warning about replacing the content of your iPhone will probably happen every time you sync.