In plain English
A system prompt is a set of instructions, set by the application developer rather than the end user, that defines a model role, behaviour, tone, and constraints before any conversation begins. It is not normally visible to users and carries more weight than a user message. It is where product decisions become model behaviour.
Why it matters
The system prompt is the main lever a developer has to make an assistant consistent and on-brand, so much of an AI product's personality and safety lives there.
A worked example
A support bot's system prompt might say: you are a friendly assistant for Acme, answer only billing questions, and never discuss competitors.
Common confusion
A system prompt strongly shapes behaviour but does not guarantee it. A cleverly worded user message can sometimes override it, which is the basis of prompt injection.