What is the Universitaly pre enrolment, step by step?

Short answer: The free official online form that links you, your university and the embassy. Every non EU student who needs a visa must do it (MUR circular).

Create an account on Universitaly with your own email. VFS Dhaka uses it to contact you (Embassy notice).
Fill in the pre enrolment for the one course you will attend; choose the embassy for your country of residence.
Upload what the university asks for, often passport and admission letter (Tor Vergata). Some universities validate Italian taught master's only after you upload their admission decision (University of Pisa).
Wait for validation, then download the summary showing enrolment and tuition fees.

This page explains official rules in plain language. I am not an immigration adviser and I do not give visa advice. For that, always use official government sources. Rules change: confirm with your university, the embassy and VFS before you act.