What it is
It is the configuration prompt that applies before anything else: it defines who the assistant is, what it can and cannot do, and what format it answers in. The user does not rewrite it in every message.
Why it matters to an agency
That is where you fix the brand voice, the constraints (do not make up prices, answer in the client's language) and the constant behavior of your agents and chatbots.
Example
"You are the assistant for [AGENCY]. Answer in English, in a friendly and professional tone, never promise results or give prices without confirming."
By the numbers
You write it 1 time and it applies to the whole conversation, while the user prompt repeats in every message. In GoHighLevel agents, a production system prompt usually runs 200 to 800 words: identity, hard rules, format and the 3 to 5 cases where it must escalate to a human. Unlike the user's message, the model treats it as authority: that is where the non-negotiable rules go.