What do the terms perihelion and aphelion mean in terms of comets?
Comets have very eccentric elliptical orbits. They originate in the outer solar system where icy bodies are common.
When a comet is at aphelion it is a frozen ice ball. It is at its furthest distance from the Sun in the outer solar system. The distance is in the order of tens or hundreds of AU from the Sun. Very long period comets have aphelia even further out.
As a comet gets closer to the Sun its ices start to melt and the characteristic tail starts to form. When it reaches perihelion it is at its closest to the Sun in the inner solar system. At perihelion its tail is most prominent.
The period of a comet is on the order of decades.