Many fitness trackers claim to be able to monitor your heart rate without the chest strap. They operate by shining a light into your wrist, which is then reflected by blood vessels passing through your veins. When your heart pumps, the blood moves through your veins at a quicker rate, causing less light to be reflected back. The tracker will then calculate your heart rate using an algorithm.
Essential reading: Best heart rate chest straps
The current trend may be to move heart-rate monitoring away from the chest and over to the wrist but accuracy at these new locations is still questionable. Particularly for high intensity workouts. If you are very serious about heart rate training, a chest rate heart rate monitor is the way to go.