Its usually a compilation of all of your core rotation evaluations, 3 note worthy characteristics you come up with, and a paragraph or two from your advisor (or whoever compiles the dean letter for you).
Dean's letter is not ERAS. ERAS is the electronic residency application service. This is like AMCAS, but for residency, and is written by the student.
Dean's letter is the MSPE, medical student performance evaluation. In brief, its a sick mixtape in the form of a standardized letter composed of 1) 3 "noteworthy characteristics" that the student writes and submits 2) boring details of your attendance (start/end date, any leaves of absence, any professionalism concerns) 3) majority of rest of letter about of your grades and with comparative charts for each clerkship with percentage of students getting pass/near honors/honors or whatever the grading scheme is with associated paragraphs for each clerkship compiled from your clinical evaluation comments. Then one or two paragraphs at the end saying "they will be a [some qualifier word] resident and we recommend them."
It is compiled by whoever does them at your school. They are often cosigned by the dean, but compiled by someone else. This is distinct from a letter of recommendation and is supposed to serve as a composite "objective" record of your performance as a student.