concept
A prompt is everything the model receives before it responds. It can be a single question, or a long document with instructions, examples and constraints.
Because a model works only from what it is given, clear prompts tend to get better results: say what you want, give the context it needs, and describe the form the answer should take.
Many apps add their own instructions alongside what you type, so the full prompt a model sees is often longer than your message.
Example
“Summarise this email in three bullet points for a busy manager” is a prompt with a task, an audience and a format.