Prompt engineering is a trick to get AI systems that understand language to generate new responses, without fully retraining them. These AI systems are first taught by reading a lot of text. Then, instead of re-teaching the whole system, engineers give it a starter sentence, called a 'prompt', and the system learns to generate responses based on the prompt. But it still uses what it learned from reading text. So it's an efficient way to get the AI to produce new responses, by just focusing it on the prompt, rather than re-teaching everything.