For a planet, comet or other celestial body moving around the Sun in an elliptical orbit, the distance between the object and the Sun changes throughout the orbit.
The position of closest approach, i.e. the shortest distance between the Sun and the planet, is known as the perihelion (from the Greek peri = near and helios = Sun). At this point in the orbit, the planet is moving at its maximum speed (Kepler's Second Law). The perihelion refers specifically to orbits aroundd the Sun, and is equivalent to the periapsis of a general orbit.
To completely specify the position of a planet, the argument of perihelion is required as one of the orbital elements.
In a strong gravitational field, the location of perihelion may advance on successive orbits. Within the Solar System, this is most easily seen in the orbit of Mercury, providing an important test of General Relativity.