Regular foam core, whether it's 1/8 or 3/16, is a foam product with white paper mounted to both sides. The paper is coated in fine white clay that is un-perceivable but beneficial as a buffering agent.
The foam material can be dented or squished if you squeeze hard enough, but this product is the most common material used for backing board in the picture framing industry. The 3/16 inch thickness is slightly more rigid than 1/8 inch foam core, and it’s also the most common thickness, but 1/8 inch foamcore can be beneficial if your frame rabbet depth is limited.
If you're doing something where it's a fine art, or it's irreplaceable, you’ll want to go with an acid-free foam core, not this regular foam core material. Watch the acid-free backing video to discover the benefits of that product.