You can use your Apple Watch and watchOS 5 to control music on your iPhone, or you can enjoy music on your Apple Watch without the smartphone.
When your paired iPhone is in range, you can use your Apple Watch to control what's playing on your iPhone. Press the side button, then tap Now Playing in the Dock. The name of the device that you currently control appears in the top-left corner.
You can enjoy music on your Apple Watch without your iPhone when you pair with Bluetooth headphones. Open the Music app, then choose what to play, including your synced content from Apple Music.
(This how-to is based on my experiences and info on Apple's support pages — where the images sometimes come from.)