The field "Density" sets the density value of the image, in DPI (dots per inch). Note that density has no effect or meaning for digital images until they are printed (the resolution of digital images is determined by their number of pixels).
Unless the image is going to be printed by software that uses this setting, it does not matter what value you enter into this field.
Setting a value for density ensures that it is set in the converted image - this may or may not be used by the software printing the image.
Enter a value of 0 if you do not want to set a value for the density.