You are now in the world of legalese. As noted the "unit" could be a room, multiple rooms, or even a building (although at BLT it is a room or multiple rooms). It is essentially like a condominim but rather than owning a entire room in the condominium building you will own a small percentage of the unit since you are buying only a small timeshare portion of the unit.
As to the importance of the unit: your portion of it is actually what you own and can later sell. However, as to reality in reservations and use, it means nothing. It would be mere happenstance for you to actually end up staying in a room that is in your unit, unless you made a specific request for a room number within your unit and the request was met.
Hereafter, your life will be controlled not by the unit you own but by points and the reservation system that is based on the use of such points. Like all other members, you will come to believe that points are the most valuable things in the universe that control all things
DVC. You will use points to reserve, you will buy more points to get to reserve more, you may bank or borrow points to get to reserve more at any one particular time, you may even someday rent your points. You will continuously think of your points as items of great value, equal to the value of diamonds or rubies. You will become Gollum and your points will be your "Precious." Then one day (it will actually takes days) you may sit down and read through the vluminous Declarations and Multi-site documents, which are two thick volumes of legalese, and you will come upon a provision that will make you gag and wonder if you have been defrauded. The little surprise that you will find is the declaration that points have no value, they are worthless, they cannot legally be transfered for money. However, I assure that once again the legalese means nothing as to your use and reserving of rooms. The points are deemed to only represent your ownership interest and thus legally you are always reserving or transfering the legal interest that you actually own but the reality for all practicable purposes will still be that those points are your "precious."