Advancing Velocity is the speed at which your opponent is moving toward you (perpendicular to Lateral Velocity). For example, a bot moving full speed straight at you would have an advancing velocity of 8, a robot moving full speed perpendicular to you would have an advancing velocity of 0, and a robot moving full speed straight away from you would have an advancing Velocity of -8. A stationary bot will always have an advancing velocity of 0, no matter which direction it's moving.
A simple formula for advancing velocity is:
double advancingVelocity = -Math.cos(e.getHeadingRadians() - (e.getBearingRadians() + getHeadingRadians())) * e.getVelocity();
e is a ScannedRobotEvent.