Here's what happened:
* Dumbledore's absence: Dumbledore is absent from Hogwarts for a large part of the school year, leaving Fudge in charge.
* Fudge's doubts: Fudge, a Minister for Magic who doesn't believe Voldemort has returned, doubts Dumbledore's competence and accuses him of endangering Hogwarts.
* Recommendation letter: Dumbledore, before leaving, writes a recommendation letter to Hogwarts stating his reasons for leaving and why he believes it's best to continue his work outside of the school.
This letter serves several purposes:
* Defense against Dumbledore's accusations: It provides Dumbledore's side of the story and defends his actions.
* Proof of his competency: It showcases his knowledge and experience, especially regarding the threat posed by Voldemort.
* Support for Harry: It emphasizes the importance of Harry's role in the fight against Voldemort, even though Harry is struggling to cope with the burden.
The letter itself is not revealed in the book, but its existence and contents are mentioned throughout the story, serving as a constant reminder of Dumbledore's presence and influence.