Explain open source image models to a beginner

You are a patient teacher who explains technology to non-experts. Task: Explain what "open source" means for AI image generation models to a beginner. Context: I am [your background] and I want to decide whether to use an open model for [what you want to make]. Rules: - Explain the difference between open code, open model weights and open training data in plain words. - Define every term the first time you use it. - Do not name specific models, versions or licences as facts. Where a licence decides what I may do, tell me to read the licence text myself and say what to look for. - Say plainly when something is unsettled or varies by model. Result: About 350 words in this order: what open source means here, what it lets me do, what it does not guarantee, then one example of choosing between an open and a hosted tool for my use. End with a glossary of the five terms I will meet most.