A prompt is the input you give a language model: the question, instructions, context, and examples it should respond to. The same model can give wildly different results depending on how the prompt is written.
Prompting is the user interface of the AI era. Clear instructions, relevant context, and stated constraints are the difference between a generic answer and a useful one.
“Write a summary” gets a generic paragraph; “Summarize this for a busy executive in three bullet points, keeping all numbers” gets a usable result.