LAN Sync is a feature of the Dropbox desktop app that speeds up syncing of files across your computers that are on the same Local Area Network. By default, Dropbox synchronizes a file to the server & then downloads that file onto the other computers from the Dropbox server. Unless your upload & download Internet speeds are super fast, this can take a long time for large files.
LAN Sync is an option that will automatically allow Dropbox to find other linked computers on the same network & synchronize the files directly over the LAN, which is way faster than uploading & then download it again.
LAN Sync can be activated by right clicking on the Dropbox Desktop icon, selecting Preferences, clicking the General Tab, then tick “Enable LAN sync”.