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()-absbearing)*e.getVelocity();
Where absbearing is the absolute bearing to your target. Without the negative sign, it could be called the retreating velocity.