The Apple Watch has eight gigabytes of on board storage, of which up to two gigabytes can be used to keep music files on the device. You can sync a playlist to Apple Watch to listen to music when the iPhone is out of range, remove songs from the device at any time and more.
How to sync Playlist to Apple Watch:
Open the Apple Watch app on your iPhone by tapping its icon on the Home screen, or use Spotlight to search for “Apple Watch”. Go to Music, Synced Playlist on the Apple Watch app. You should see a screen listing all the playlists available in the iOS Music app. Now tap the playlist you want to synchronize to your Apple Watch. When a Sync Pending message appears, your playlist will begin syncing from the iPhone to your Apple Watch. If it reads “Syncing…”, your music has just begun transferring to the Apple Watch. After the playlist has been synced to the Apple Watch, its name should appear next to Music, Synced Playlist in the Apple Watch companion app.You’re advised to wait until the sync is finished before using your Apple Watch. To see the sync progress, open the Apple Watch app on your iPhone and tap Music.
In order for the music files to transfer over to the Apple Watch, the device must be attached to power and Bluetooth must be enabled on your iPhone. As evidenced on the screenshot below,
the Apple Watch app itself makes it pretty clear that the playlist syncs only when your Apple Watch is on its charger.To charge your Apple Watch, simply connect it to its Apple Magnetic Charging Cable. To enable Bluetooth on your iPhone, open the Settings app, navigate to the Bluetooth section and make sure Bluetooth is turned on.Disabling Bluetooth, or walking outside Bluetooth’s range of up to 30 feet (about 10 meters) while your music is being transferred to the Apple Watch, will temporarily pause the sync. Once both devices have re-established Bluetooth connection, the sync will automatically resume.