This is not really the angle, just an attempt to simulate the angle. It is the ratio between rise(price) and run(Number of Candles) also called Tangent of the angle. The original code used a factor to get a larger value because the run is usually a value from 2 to 6 while the rise might be much larger, especially on daily chart GBPUSD where 6 days can have a movement of 1000 pips. The resulting tangent would be 6/1000, a very small angle.

The indicator shows when the "angle" is increasing, decreasing or so small that the trend is flat.

Robert