A seed number generates the random starting noise used in the beginning of an image generation. Using the same number across multiple prompts will use the same starting noise.
How to know what the Seednumber is?
React with the envelope emoji ✉️ and go to your direct messages from the Midjourney Bot
How to iterate on a prompt using the same seed number?
--seed yourseednumber at the end of your imagine prompt.
--seed 23043 to get more stylized results on your first apple imagine prompt with seed 23043