What is a context window and why does it matter?

The context window is the maximum number of tokens (input + output) a model can consider in a single request. Anything beyond it is truncated. It matters because it bounds how much document, conversation, or code the model can reason over at once — and because models tend to use the start and end of a long context better than the middle ('lost in the middle'). Bigger isn't automatically better; relevant, well-placed context beats sheer volume.