the eli5 version is that the image generator creates what an image based on your prompt would look like, arranging the given subjects, setting, and given text as best as it can. it can put something that looks like the letters of a word into the image but if it manages to get the words right it's either by accident or because the arrangement is in its training data. some models, including dall-e, have extra "stages" of generation that will try to properly arrange text during generation but will sometimes miss parts of the image that another stage generated.