Compared to your external layer of fat (which may be annoying but is much less dangerous), visceral fat is wrapped around organs and is metabolically active, contributing not only to weight gain, but to the incidence of chronic disease (heart disease, diabetes, cancer, Alzheimer’s….the list goes on.)
The thing with visceral fat is that it’s perfectly possible to be thin, with a relatively high level of visceral fat. It’s also possible to be larger, with a relatively low level of visceral fat (and chronic disease risk.)
The image below shows us that you can’t judge health risk based on overall weight alone. We have the MRI cross section of eight different men, all with the same waist circumference. As you can see, the external layer of fat varies, but the visceral fat (the white marbling wrapped around the organs) increases in volume as the images progress. The first image (showing 0.5L of visceral fat) represents someone who is healthy at that size, the last image (showing 4.3L of visceral fat) represents someone who is unhealthy at that size.