A system prompt is a set of standing instructions given to an AI by the application running it. It can describe the AI's role, how it should respond, or what limits it must follow.
Think of it as house rules for a helper. A one-time request might say “explain this bill,” while the standing instruction might say “use plain language and do not make payments.”
The person using an app may not see these instructions, and different AI products handle them in different ways. A system prompt can guide a response, but it does not guarantee that the AI will always follow the rules perfectly.