Support vector machines - separating hyperplane question

It is the equation of a (hyper)plane using a point and normal vector.
Think of the plane as the set of points P such that the vector passing from P0 to P is perpendicular to the normal

Check out these pages for explanation:

http://mathworld.wolfram.com/Plane.html
http://en.wikipedia.org/wiki/Plane_%28geometry%29#Definition_with_a_point_and_a_normal_vector

Imagine a plane in a 3d coordinate system. To describe it, you need a normal vector N of that plane and the distance D of the plane to the origin. For simplicity, assume the normal vector has unit length. Then the equation for that plane is x.N - D = 0.

Explanation: x.N can be visualized as a projection of x on the normal vector N. The result is the length of vector x parallel to N. If this length equals D, the point x is on the plane.