This challenge is about writing a letter to someone who deserves your gratitude but who you think has not yet received that gratitude or has received it insufficiently. In principle, this can be anyone from your network (friends, partners, family, classmates, etc.), but preferably someone you can meet with, be it in person or online.
Research in the field of positive psychology shows that the Gratitude Letter can be a great gift that can improve both yours and your recipients mental health and well-being.
You can find more information in this video: