Gidday Ratty, Here's my understanding of this; as you may already know light travels in 'waves' of varying wavelengths. Light toward the red end of the spectrum has a longer wavelenth than that at the violet end of the spectrum.
When light enters the telescope and strikes the mirror it is reflected back up the tube (in the case of a reflector type) to a point known as the focal point. A spherical mirror (one which has a surface shaped like part of a perfect sphere) tends to bring light of different wavelengths to slightly different focal points.
This causes an optical effect known as spherical abberation which can lead to unnatural colour casts and an effect called coma, which produces elongated images of points (the stars look like little comets). Using a parabolic mirror (one shaped like a parabola, which is the shape you get if you slice vertically down thru a cone) tends to alleviate these effects by focussing all wavelengths of light at virtually the same focal point.
I understand there are some telescopes with mirrors that have a modified parabolic shape (Meade RCX400 - Ritchey-Chretien) that virtually eliminate these effects altogether BUT they come come with a mind-numbing price tag!
Hope this helps