The source is a transcript of a YouTube video by Firebase explaining how to use their Genkit platform to generate images and text using large language models (LLMs). The speaker demonstrates how to use Genkit to call upon LLMs such as Gemini 1.5 Flash, which can accept images and videos as input, and Imagen 3, which can create images from text prompts. The speaker also highlights how these LLMs can be chained together, so that an image generated by Imagen 3 can be used as input for Gemini 1.5 Flash to create a story. The example showcases the flexibility and ease of use of Genkit for multimodal generation with LLMs.