When a company releases a model's trained parameters for anyone to download and run. "Open weights" is more accurate than "open source" because most released models don't include training data or training code — you get the finished model but not the recipe. Llama, Mistral, and Qwen are open-weights models.
Why it matters
Open weights mean you can run AI on your own hardware with full privacy — no API calls, no data leaving your network. The trade-off is you need the GPU resources to run them and you're responsible for safety.