Think of a 3D mesh as a wireframe skeleton. A "preset" is a pre-constructed skeleton. Instead of building a 3D sphere from scratch using code or complex modeling software (like Blender or 3ds Max), Photoshop CC 2015 provides you with a library of ready-to-use geometric shapes.
When you navigate to the menu bar and select , a dropdown list appears. These are the native presets included out-of-the-box: 3d mesh presets for photoshop cc 2015