SDSS records information in long continuous strips across the sky, one strip for each night’s observations.  For analysis, astronomers divide this data up in a number of ways.  A Field is one chunk of an observation stripe that can be downloaded to your computer to be analyzed.  It is still a large image measuring 2048 pixels long by 1489 pixels wide.

Each field overlaps with the adjacent fields by 128 pixels.  This overlap represents duplicate data, not a separate observation.  This overlap becomes obvious if you zoom out a few clicks from your target.

Astronomers could be interested in seeing the field boundaries if they are studying a particular region of the sky.  They might want to restrict their study to objects observed in the same night or download an image that contains a galaxy of interest and all its neighbors in one image.