Context window
The context window is how much text a model can take into account at once — your instructions, the conversation so far, any documents you pasted, and its own reply. It is measured in tokens. Anything beyond it is not seen.
In one line, for a 12-year-old
The context window is the AI's short-term memory: how much of the chat it can keep in mind at one time.
An example
In a very long chat, an assistant may "forget" a detail you gave at the start because it has slipped out of the window or been summarised.
Why it matters to people
A bigger window lets you work with whole contracts or reports, but it also means more of your information sits in one place. Paste only what the task needs.