The Dot Product 
(x1, y1, z1)  .  (x2, y2, z2)  =  (x1)(x2) + (y1)(y2) + (z1)(z2)

the dot product is in the equation to find the plane.  
I can't remember exact meaning of dot product (cross is normal vector), 
but dot is a projection or something (words of Jon).

It's needed for the plane equation



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({x, y, z}-point).normal
                       
point is the point which is used for the start of the 2
vectors which are made from other 2 points to the point. 
Then cross product of those vectors for normal.

(words of Jon again)