Photos synced with iCloud

I am intending to move the original photos from my iPhone to iCloud.

In Photos Settings, under iCloud Photos, I toggled the button that said

"Automatically upload and safely store all your photos and videos in iCloud so you can browse, search, and share from any of your devices."

But when I did, I got this message:

"Photos and Videos Synced from Finder will be Removed - Syncing photos via Finder is not supported when iCloud Photos is turned on.

xx,xxx photos and videos will be removed.

Remove Photos and Videos / Cancel "

Can someone explain to me exactly what the warning meant?

I have 2T of iCloud memory with 90% free storage...

Thank you

Booster E.