In plain English
Most popular chatbots run on one company's computers, and you can only use them through its app. With an open-weight model, the finished model itself is released. A business can run it on its own computers, keep its data private and adjust it for its needs.
Example
A hospital downloads an open-weight model and runs it on its own servers, so patient notes never leave the building.
In short
Download the model itself. Run it and adapt it yourself, not just through an app.
Why it matters
Open-weight models give businesses more control over cost, privacy and customization, and keep pressure on the biggest AI labs. They're also at the center of the debate over whether powerful AI should be freely available.
Common confusion
Open-weight vs. open source. Open-weight means you get the trained model. Fully open source AI also shares the training code and details of the training data, with few limits on use. Many open-weight models come with license restrictions.