System prompt
A system prompt is a set of standing instructions given to a model before the conversation starts — its role, tone, rules and limits. Apps use them to shape the assistant; many chatbots let you add your own "custom instructions" too.
In one line, for a 12-year-old
The system prompt is the secret rule sheet the AI reads before it talks to you.
An example
A library chatbot's system prompt tells it to answer only about library services, in plain language, and to suggest a librarian for anything else.
Why it matters to people
Hidden instructions shape what a chatbot will and won't say. Organisations using AI with the public should be open about its rules and limits.