It means that you can only sync one catalog a time between the desktop/laptop and mobile device.
"Switch to syncing this catalog"......
If you select the above option, then it will remove all the existing images from the Lightroom mobile from the previous catalog and start the new sync with this one.
1 person found this helpful
Your problem is that you should have copied your catalogue from the old computer to the new one. Instead you seem to have created a new catalogue.
So choose Don't Sync, go back to the old computer and find that synced catalogue.
What if you do not have access to that file and want Lr Mobile to sync to you new Mac Book Pro
In general, you start all over again. So you accept the dialog message's "Switch to syncing this catalog", your stuff currently on LrMobile disappears, and you learn to back up your catalogue. Hard lesson, but you won't make the mistake again.
Syncing a new/different catalog will cause your old LR mobile pictures to be discarded so that LR Mobile matches LR Desktop.
Assuming the old images are in the new catalog, they will be synced up again to LR Mobile.