What's your question, exactly? What's the Dean's Letter? Every med student has their dean write them a composite "Dean's Letter," which is a summary of all your med school accomplishments - grades, evaluations, board scores, extracurriculars, etc. This letter is released to ERAS on the same date every year - I forget when, exactly...many residency programs start inviting applicants for interviews after they receive this letter.
I don't know how other schools do it, but my school (LECOM) gave us some paperwork to fill out for the dean asking us questions about what extracurricular activities we had been in at school, words we thought described ourselves, etc. to help her write the Dean's letter. The school releases the Dean's letter through ERAS on November 1 (I think...I'm not sure as I didn't apply through ERAS).
And "pre-match" refers to being offered a position outside of the match (i.e. signing a "pre-match" contract).