A job offer rejection letter is a letter written by a person to decline a job that has been offered to them. It is drafted on plain paper, or sent by email, and signed by the person turning the offer down.
The content is brief and deliberately so: a thank you, a clear statement that the offer is being declined, and nothing that invites a negotiation you do not intend to have. A reason is optional and is usually better left general. "I have accepted another role" says everything that needs saying.
It matters more than its length suggests. Recruiters keep records, hiring managers move between companies, and the shortlist you were on is small. Declining well is the cheapest professional courtesy there is.