The JPEG settings dialog box appears when you press the JPEG settings... button on the Save Image As dialog box. It lets you control attributes of the JPEG image file.
The text field and its associated slider control the tradeoff between JPEG image file size and image quality. Lower quality settings tend to reduce file sizes.
NOTE: Extremely high quality settings may produce large files with minimal improvements in image quality. In general, there may be diminishing returns for values higher than 75%.