What EXACTLY does Syncing do?

If I sync my iPhone to iTunes on my laptop, what will happen? Will things get erased, will I still have everything the same, I need to know! Please try to explain it to me as simple as possible. As soon as possible

Syncing means connect something each other.
If you do sync on iTunes. ITunes will connect your iPhone. If you want to upload pic, song, videos, and everything you have to do syncing your iPhone. Warning: if you upload new pic, songs and videos iTunes will automatically erase old pics. Songs. And videos. So I recommend iTools or ifunbox. I have used iTools. It is very easy to manage your iPhone or any apple devices. Download iTools it no need to install. First install iTunes then open iTools.

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