It's basically a form that you sign which waives your FERPA rights and allows your school to provide information regarding your academic and disciplinary record to the requesting institution.
I'm just curious as to which schools require this, as I graduated a while ago and it may be a hassle to deliver such a form to my undergrad school.
I've only heard of WashU requiring this, but I suspect there are other schools which also require the dean's certification (perhaps once you have been accepted).