Tells you, to put it in really basic terms, how planar a polygon is. If you were optimising a facade for example, and needed to evaluate each panel to ensure it is flat (glass/fabrication for example), this node can be used to perform the test.
I get understand, Thank you 
, but I think it will not be a deviation in any panel consists of four points, am I right ?
Yes, a surface of 4 (or more) points can have deviation and be warped. A surface of 3 points however cannot.
Can you show me how a surface of 4 (or more) points can have deviation to get better understand.
This might help you to understand:
Draw a plane using Plane.ByThreePoints
Point 1 = Origin
Point 2 = Plane Point
Point 3 = X axis
Planes have an origin, normal and and X and Y axis.
If a fourth point, or more, is introduced it cant be used to construct the plane since all of its constructors are satisfied by a maximum of 3 points. Therefore any other point beyond three can be moved ‘out of plane’.
If we now go back to the facade example with panels, if this fourth point is moved ‘out of plane’ and the polygon is constructed using the four points, it becomes warped and is therefore not planar. The only way to guarantee planarity is to use three points as @Daniel_Woodcock1 said.
