In plain English
A prompt is the input, the text, question, or instruction, that you give an AI model to elicit a response. Because the model only has what you give it to work from, the clarity and structure of the prompt strongly shape the quality of the output. Prompting well is mostly a communication skill, not a technical one.
Why it matters
The model has no background on you or your intent beyond the prompt, so a clear, specific brief is the single biggest lever you have over the result.
A worked example
'Write an email' produces something generic, while 'Write a warm two-line reply declining this meeting and proposing Friday' gives the model what it needs.
Common confusion
There is no secret magic wording. Good prompting is just clear, specific instruction, and the same prompt can give slightly different answers each time.