what does it mean on icloud photos on my phone "photos synced from finder will be removed.Syncing photos via finder is not supported when icould photos is turned on. 5,977 photos will be removed."it gives me a choice to remove photos or cancel

Don't delete them from your devices. iCloud is a sync service, not a backup service, so it keeps an identical library accross all your devices. What you delete on one device gets deleted in iCloud and on all other devices.

Instead, what you need to do is:

On your phone : In icloud photos settings, select "optimise iphone storage"

On your mac in Photos preferences>iCloud select "optimise mac storage"

This will replace the full size images on your devices with much smaller preview images. Full size images will only be stored in iCloud. If you want to use one of the images (eg to edit it, or export it) then the full size version is automatically downloaded.

One thing to be aware of, on the mac, the large images will only be removed when you start to run low on space. If you want to get it to remove all large images immediately, this user tip explains how.

https://discussions.apple.com/docs/DOC-250002222

It means the photos you have synced from your mac to your phone using finder will be removed from your phone.

If you want to keep them on your phone, before enabling iCloud, you can select them all and choose "copy to camera roll"

(It might also be "move to camera roll" I can't check, because I already use iCloud.

Alternatively if all these photos are still on your mac, then you can allow them to be deleted from your phone, and sync your mac/phone vial iCloud, and they will be put back on your phone that way.

(I'm assuming you have subscribed to enough iCloud space for all your photos)

Welcome 🙂