Context window
The maximum amount of text a model can hold in mind at once, including your prompt, the history and any attached documents.
Everything the model can 'see' when answering has to fit in the context window: system prompt, conversation history, retrieved documents and the current question.
Bigger is not automatically better. Answers measurably degrade when a long window is filled with loosely relevant material, so curating what goes in beats maximising how much goes in.