There are many different kinds of "projection" but using the word alone typically means the "orthogonal projection" where the projected point lies on the line through the given point perpendicular to the plane.
Here the plane is given by ax+ by+ cd= e so ai+ bj+ ck is a vector perpendicular to that plane. The line through (x0, y0, z0) perpendicular to that plane is x= x0+ at, y= y0+ bt, z= z0+ ct. Replacing x, y, and z in the equation of the plane gives one equation to solve for t where the line intersects the plane. Replacing t in the equations for x, y, and z gives the coordinates of that intersection point, the projection point.
For example, suppose the given point is (3, 3, 1) and the plane is x+ y+ 2z= 2. Then the line through the point perpendicular to the plane is x= 3+ t, y= 3+ t, z= 1+ 2t. Putting those in for x, y, and z in the equation of the plane gives (3+ t)+ (3+ t)+ 2(1+ 2t)= 8+ 6t= 2. 6t= -6, t= -1. The projection point is x= 3- 1= 2, y= 3- 1= 2, z= 1- 2= -1, (2, 2, -1).
