The seeing of an image is a measure of its quality or sharpness. Because stars are so far away and are so small (relative to a galaxy for example), they appear as points of light (point sources) in our images. We measure the seeing as the angular extent of a point source in the image.
The seeing of an image should correspond to either the diffraction limit or the atmospheric seeing, whichever is greater. Therefore, three factors affect the seeing of an image:
Here is an example of how poor seeing blurs an image and affects our ability both to distinguish separate objects and to see structural details. The first panel shows the image of a distant cluster of galaxies obtained by Martha Haynes and Katie Jore using the COSMIC imaging camera on the Hale 5m telescope at the Palomar Observatory. In the other images, we have artificially blurred the images by an increasing amount. Notice the difference in your ability to discriminate details when the seeing is degraded. Notice too that some objects practically disappear when the seeing is poor.