There is no headphone jack, but it supports Bluetooth wireless headsets.
Apple Watch has a built-in music app, which can directly control songs in the iPhone "Music" app.
If you want to play completely on your watch without your phone, you can use the "Apple Watch" program on your phone, select the "My Watch - Music" option, and synchronize the songs in your iPhone to your watch (total song capacity No more than 2G).