Vertical Ratio is the relationship between Vertical Oscillation and Stride Length.
Vertical Oscillation is the amount your body bounces when you run, and Stride Length is the distance you travel with each step. Vertical Ratio is the relationship between the two. If you train to have lower Vertical Ratio numbers, you will waste less energy bouncing while covering longer distances with each stride. This metric can be used to achieve a more efficient running form.
To record Vertical Ratio you can wear a Running Dynamics capable Garmin watch like the 935, and an additional Garmin sensor (the HRM-RUN or HRM-TRI chest straps, or the RD Pod). Another option is to use the Stryd footpod with a compatible watch from Suunto, Polar, etc.
Find it in SportTracks
If you use the required hardware, you can find Vertical Ratio in your Workout Detail charts: