A fixed week is when you buy accommodation at a resort for one specific week. The upside is that your week is guaranteed. You own it and the place is yours.
Fixed week timeshares as originally conceived, conveyed the rights specific to a resort, in an identified property and at a certain period in time (generally 7 days). Thus, the rights were “fixed”.
The downside? If you can’t get away during any given year for that first week you may forfeit your time without compensation. (Unless you joined an exchange club or found someone at the resort willing to trade with you.)