You ask an AI to help you prepare for an interview. You get back twenty generic questions you could have found on any blog.

You ask again. This time you mention that you are an entry-level developer, that you know the MERN stack, and that you want the answers in a table. Suddenly it is useful.

Same model. Same knowledge. Different prompt.

That gap, between the generic answer and the useful one, is what prompt engineering is about.

Prompts, and why they matter