In general relativity, this remaining precession, or change of orientation of the orbital ellipse within its orbital plane, is explained by gravitation being mediated by the curvature of spacetime.
What is orbital precession?
Precession—the change in orientation of the Earth’s rotational axis—alters the orientation of the Earth with respect to perihelion and aphelion. If a hemisphere is pointed towards the sun at perihelion, that hemisphere will be pointing away at aphelion, and the difference in seasons will be more extreme.
What is precession of the perihelion?
As the planets describe their orbits, their major axes slowly rotate about the Sun, in the process shifting the line from the Sun to the perihelion through an angle ϕ during each orbit. This shift is referred to as the precession of the perihelion.