Prompt
A prompt is the message you give an AI tool to tell it what you want.
In a simple example
You type: Make a one page website for my cake shop, with a menu and a button to order. That sentence is the prompt.
Why it matters to you
The clearer your prompt, the closer the first answer is to what you wanted, and the less time you waste.
Do not confuse with
A prompt is what you write to the AI. The answer it gives back is called the response or the output.