The car is rotating mid-air because of the instantaneous torque acting on the car's center of mass. When the angle is zero, then the car is oriented parallel to the horizontal axis. For negative angles, the car front points up, and for positive angles, the car front points down.
It's a standard way to measure the angle in polar coordinates. The zero is along the x-axis to the right, 90° is along the y-axis upwards, and so on. Read more