What it is
A prompt is everything you write to the model: the task, the context, the format and the constraints. A good prompt turns a generic answer into one that is ready to use.
Why it matters to an agency
Proven prompts are reusable assets: you save them, fill them in with client data and run them again and again with consistent results.
Example
"Act as a sales closer for an agency. Give me 3 follow-up variants in a table, in English, without making up prices."
By the numbers
A useful prompt usually has 4 pieces: role, context, output format and constraints. Adding 2 or 3 examples of the expected result (few-shot) is the improvement with the most effect for the effort. Unlike a Google search, the prompt does not look anything up: it instructs, and the model obeys what is written, not what you meant.