Stable Diffusion is an open-source generative image model whose code can be freely run, studied, manipulated and reimplemented. It is fundamentally distinct from it's comparably capable predecessors which were not open-source.
This site hosts collections of output images from collaborations between visual artists and Stable Diffusion by way of textual inversion training sessions. The code and resources used for these collaborations is all open and available for free use.
Astronaut horse refers to the ubiquitous image of an astronaut riding a horse that often acts as a benchmark for the capabilities of generative image models.