It's meant to provide an indicator of the split in power production between your left and right legs - an indicator of asymmetry in power output. It's a pretty simplistic indicator of what's going on with pedalling forces, and masks over much detail. It's also not all that clear whether it's of much value.
Power balance is now available via compatible ANT+ head units paired with power meters that supply power balance data (e.g. some Quarqs, Power2Max, Garmin Vector, Rotor). Other power meters can also provide such data via their own proprietary data recording systems (e.g. SRM, Polar/Look, MEP, Axis Cranks), and of course some indoor trainers have provided similar data for many years (e.g. Computrainer and Wattbike).