System prompt
A system prompt is a hidden message that tells an AI how to behave, before the user says anything.
In a simple example
A shop helper is told: You are friendly, you answer in Arabic or English, and you never promise a price. Customers never see this.
Why it matters to you
It sets the rules and the tone of your assistant, so it behaves the same way for every customer.
Do not confuse with
The system prompt is written by you, the builder. The ordinary prompt is written by the user.