A prompt is what you type to tell the AI what you want. It can be a quick question or a few lines with a role, a task, some context, and the shape you want back. The AI answers based on what you put in, so the prompt is your main steering wheel.
The rule of thumb: a vague prompt gets a vague answer; a clear one gets a useful answer. You do not need magic words, just enough detail that a smart new colleague could do the task.