Live sync creates a two-way connection between a MEGA cloud folder and a local folder. Files that appear in MEGA are downloaded automatically. But files deleted locally will also be deleted from MEGA.
This is different from Download, which copies files from MEGA without ever deleting anything from the cloud.
If a file is deleted locally (even by accident), MEGA sync will also delete it from the cloud. Deletions propagate in both directions. This cannot be undone.