Yes, you can use the generated images commercially as long as you follow the content policy and do not use them for anything illegal.

Each credit allows you to generate a single 512x512px image at 30 steps with Euler sampler (Default settings for SD 1.5). The cost of generating an image varies depending on the parameters and tools you use. It's displayed on the "Generate" button, so you can always see how many credits a certain generation will cost.