The system prompt is not your question, it is the rulebook. This is where you say who the model is in this situation, what it may and may not do, what tone to answer in, and what to do when it does not know something. Every user message is read against it — which is why a bad system prompt shows up in every single answer.
Three dials are worth seeing together: the system prompt (the standing frame), the prompt (the task at hand) and temperature (how varied the answer should be). Two of them you use daily without noticing, and the third one most people set wrong.
The most common misconception is that the system prompt is a security boundary. It is not: it is an instruction, one that a cleverly worded input can overwrite, and one the model will not necessarily follow anyway. What actually constrains is permission and process, not text.
The three dials and how to set them properly are the subject of Prompt, System Prompt, Temperature: The Three Dials, a separate article. Directly related terms: prompt, temperature and guardrail.