The amount of downward pressure that the strings exert onto the bridges. The pressure is proportional to the amount of upward inclination of the strings at the bridge(s). A proper downbearing is necessary for tone of good volume, projection, and color.
How to measure downbearing:
Place the three-pronged gauge on the middle string over the bridge pins. The middle prong should rest on the bridge, while the outer prongs should rock back and forth on the speaking and non-speaking sides.
If there is no downbearing, the gauge will not rock: