A prompt is everything you give an AI assistant before it answers. Usually it is a question in plain language, sometimes with extra detail about your situation or the form you want the answer in. Improving prompts is called prompt engineering. In its prompt engineering overview, Anthropic recommends first defining what a good answer looks like and how you will test for it.
How does a prompt differ from a keyword?
A keyword is often short. A prompt can be a full question, including circumstances and wishes.
Search: dog groomer Haarlem
Prompt: My labradoodle gets very anxious around clippers.
Which groomer in Haarlem works calmly and without rushing?Why does it matter?
For your website, this means pages should also cover specific situations like these. For measurement, the prompt is your yardstick: you can only compare answers over time if you use the same prompts.
Common mistake
Wording the question slightly differently at each measurement, or measuring in an account with personal settings. OpenAI writes in its help page on search that ChatGPT can use saved memories and a general location when it rewrites a search. Record the environment you measure in.
More in How do you measure your visibility in AI answers? and under share of voice.