These represent a low-level handle to a VIPS image structure. You can make them with the vips_image builtin, and you can pass them as parameters to VIPS functions. The Image class is built on top of them, see §6.12.3.
As an accident of history, nip2 also lets you do arithmetic with them. This will probably be removed in the next version or two, so it's best to go through the higher-level Image class.