Masking involves setting the pixel values in an image to zero, or some other "background" value. Masking can be done in one of two ways:
Using an image as a mask. A mask image is simply an image where some of the pixel intensity values are zero, and others are non-zero. Wherever the pixel intensity value is zero in the mask image, then the pixel intensity of the resulting masked image will be set to the background value (normally zero). You might, for example, create a mask image using the Particle Analysis tool.