Speckling is an artifact of lossy compression that causes pixels which should be transparent to display in the output.
For example, you may specify 0,0,0 as the alpha band values for an RGB image so that all of the black pixels are made transparent. But in compression, the values of some pixels undergo slight changes in value. The difference is not be visible to the eye, but a pixel whose RGB values are 0,1,0 or 0,0,1, while it may still look black, no longer matches the specified transparency profile and so it will not be rendered transparent. Where this happens, speckling can appear in the collar regions of an image such as a DOQQ. In this case, select the Despeckle Output check box to ensure that all of the pixels that should be transparent are treated correctly.
For more information and to manually set despeckling options, see The Despeckle Tool.