In almost every programming language there some sort of math module with various trigonometric functions including functions called atan and atan2.
Now everybody knows atan, it’s just inverse tangent, arctan, just as we know it in math. If we have vector v with coordintes [x,y], and we calcualte atan(x,y), we cannot really tell in which quadrant the vector v is in. Absolute value of this result just tells us the smallest angle between the vector v and X-axis.
Atan2 is nothing else just more comfortable version for atan. Result of atan2 returns angle between vector v from the “angular start” on unit circle.
Picture is worth thousand words
Compare what angles in radians is returned by atan(x,y) compared to atan2(x,y)
Here, calculating atan(x,y) would result in value approximately around -pi/6 showed on the picture as angle alpha.
In this case, calcualting atan2(x,y) would result in value around 5pi/6, showed on the picture as angle alpha.