The “Seed” is a number from which Stable Diffusion creates the blueprint of the image (in actual term: noise). If seed is -1, it creates a random seed. Using the same seed with a slightly different prompt will yield a similar image.
The cfg (classifier free guidance) scale is a parameter that controls how much the image generation process follows the text prompt. The sweet spot is between 5-13. Too high will frequently make strange images. And if you are into strange images, go ahead and put in a high number. A good laugh never hurt anybody 😀