Advancing Velocity is the speed at which your opponent is moving toward you (perpendicular to Lateral Velocity). When you stay still, RamFire usually has an Advancing Velocity of 8 and MyFirstRobot usually has an Advancing Velocity of 0. A robot moving straight away from you would have an Advancing Velocity of -8.
A simple formula for advancing velocity is:
double advancingVelocity = -Math.cos(e.getHeadingRadians() - (e.getBearingRadians() + getHeadingRadians())) * e.getVelocity();
e is a ScannedRobotEvent.