Basically it turns stars into fuzzy oblongs instead of points, particularly towards the edge of the field. It is generally associated with poorly made optics or collimation errors. It also reduces lunar and planetary contrast. It usually shows up more as speed and aperture increase. Achromat doublets in refractors help reduce this aberration and more expensive, sophisticated designs like apochromats and those using fluorite lenses can virtually eliminate it.
Collimation is critical for achieving optimum results.
Poor collimation will result in optical aberrations and distorted images. The images seen produce a V-shaped appearance. The center of the field may be sharp and in focus but the edges are out of focus and vice versa. The larger the field of view, the larger the area of the sky you can see. The longer the focal length of the telescope, generally the more power it has, the larger the image and the smaller the field of view.