Each (rotation symmetric) lens has two principal planes. These (hypothetical) planes are perpendicular to the optical axis and are the planes on which light beams parallel to the axis coming from infinity seem to bend (and then go through the respective focal points).
The image side primary plane is formed where a light ray parallel to the optical axis enters the first lens of a lens system and intersects with the corresponding ray leaving the last lens element.
The object side primary plane is formed where a light ray parallel to the optical axis enters the last lens of a lens system and intersects with the corresponding ray leaving the first lens element.
In a single thin lens
the two principal planes merge and can be approximated by the center of the lens
The intersection of the principal planes with the optical axis.
The two principal points of each (symmetric) lens belong to the Gauss points.
The distance between the principal point and the corresponding focal point is called focal length.