Bodies within the Solar System orbit the Sun in paths that are elliptical in form to a greater of lesser degree, with the Sun at one focus of the ellipse. This means that the body's distance from the Sun will vary, sometimes approaching more closely, and sometimes receding farther away. The closest point to the Sun for any orbiting body is known as its perihelion.
For planets like Earth, the elliptical shape of their orbits shows little variation from a circle (that is, it has low eccentricity) and so the planet's distance from the Sun remains relatively constant. At its perihelion point, which it reaches early each January, Earth is 147.1 million kilometres from the Sun. At its aphelion in July - its farthest distance, opposite to its perihelion - it reaches a distance of 152.1 million kilometres. That equates to a difference of just five million kilometres (or less than 400 planetary diameters).
Other planets have more eccentric orbits, leading to greater variations in their distance from the Sun. One notable example is Uranus, whose perihelion point is no less than 271 million kilometres closer to the Sun than its most distant aphelion position (or more than 5,300 planetary diameters). These variations are even more pronounced for many dwarf planets like Pluto, and reach extremes with the long-period comets. These comets pursue highly elongated orbits that carry them far into the depths of the Solar System, but periodically they will fall into the inner Solar System and make a close approach to the Sun. The perihelion points for comets like these are often well within Earth's orbit.
The term perihelion derives from two Greek roots, peri- 'near' and helion 'sun', and as this suggests, it applies specifically to the closest approaches of planets, asteroids, comets and other bodies in orbit around the Sun. The equivalent but broader term for the closest approaches of bodies in orbit around any star is periastron. For bodies in orbit around any other object of any kind, the general term is periapsis (sometimes seen as periapse or pericentre).