It means that the images have a vector mask controlling which areas are visible (acting as a crop). These aren't affected by a RFS action as the vector data is as small as it can be, but the warning is shown because if the underlying bitmap image is resampled, there may be some issues around the edges of the mask (e.g. if the mask very accurately cuts out a shape from a background, some pixels of the background may appear if the image PPI is reduced).
I see, we are trying to reduce the file size of 13 Mb. I've tried printing/PDF smallest file size, reduce file size and optimize. None are reducing the size. Is there anything else I can do?