What is the full definition of changing phase angle of the camshaft

In the cam-phasing variable valve timing varying the valve timing by shifting the phase angle of camshaft

what is means the phase angle of camshaft ????
thanks

Since a camshaft turns thru a 360 degree circle, the cam lobe goes from a low point to a high point (where valve is fully opened) and if you were to graphically illustrate this cam action it would look like a sinusoidal wave, with max amplitude when lobe has actuated the valve fully and zero (x-axis) when cam is off the lobe.

Phase shift is if you were to move this sinusoidal graph over by say 20 degrees so that there are now two waves on the graph. What phase shifting does is make the cam lobe open earlier or later than normal. Normally the camshaft turns in direct proportion to the crankshaft. The phase angle shift allows the cam to move on a separate shaft from the crankshaft connection.

It is very similar to advancing the spark on a distributor where the spark occurs sooner than TDC in the cylinder to allow the gases to start burning before the cylinder gets to the top of the chamber

With phase angle shift, it allows the cam to move it's lobe ahead of the cylinder's bottoming or topping path to allow for the fuel and exhaust to burn more completely and let the exhaust out faster as the RPM's increase