What is Open Source?
Models whose code and/or weights are publicly available for anyone to use.
// Explanation
In AI, open source means the model's weights (what it learned during training) are public and downloadable. You can run them on your own server, modify them, build on top of them. Meta's Llama and Mistral are the best-known examples. The counterpart is proprietary models like GPT or Claude, which can only be accessed via API.
// Example
Meta's Llama 4 is open source: you can download and run it on your computer (if you have a decent GPU). GPT-5 is not: you can only use it by paying OpenAI.