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?
Use --seed yourseednumber
at the end of your imagine prompt.
Example
/imagine apple --stylize 10000
--seed 23043
to get more stylized results on your first apple imagine prompt with seed 23043