The prompt is your input to the AI. It can be a question, an instruction, a half-finished draft, a pile of background material, or all of those together. The AI's answer is shaped almost entirely by it.
For a while, "prompt engineering" was treated as a mysterious new profession, with tricks and magic phrases. Most of that has aged badly. What actually makes prompts work is what makes any brief work: say what you want, give the relevant background, show an example of good output, and state the constraints. If you have ever briefed an agency or a freelancer, you already have the core skill.
Why you keep hearing it
Because it is the basic verb of using AI ("just prompt it"), and because the debate about whether prompting is a real skill refuses to die. The emerging consensus: clear communication and good context matter far more than clever wording (see context window).
What it means for you
Treat prompts like briefs. The quality of what you get out tracks the quality of what you put in, and vague requests produce generic answers. When an AI disappoints you, before blaming the tool, reread your prompt the way you would reread a brief that came back wrong.