There are many online resources on the technical aspects of multiple exposures, but if you have a basic understanding of negative and positive values it is enough.
White, in terms of exposure, represents burnt out data. Once burnt out, that part of the image is gone forever, you will not be able to bring it back or overlay it with anything. Black represents areas where nothing was exposed and these areas are perfect for overlaying the second exposure. The second exposure will “fill in” the darker parts.
When shooting with film one also has to keep in mind that the combined exposures could result in the overexposure of highlights in the final image. This results in the need to underexpose each image in order to end up with correct exposure for the final image. The good news with digital is that as long as “auto gain” is switched on with Nikon or “multi-exposure control” is switched to “additive” on Canon, the camera will sort this out for us.