This radio button sets the resolution of a false-color image saved with the File-Save False Color Image or Process Multiple Files options. "Low" resolution corresponds to a 512x512 image, identical to that displayed on the screen. "Medium" resolution corresponds to a 1024x1024 image. A "High" resolution image is saved with the same number of pixels per side as the larger dimension of pixels in the original data set--i.e., a 1024x2048 data image would be saved as a 2048x2048 square pixel array.
Note that higher resolution images take longer to save and may consume substantially more disk space.