Some like you said, are basically an reservation to for accomodations for a specific time period every year at a specific place.
But there are others that are basically "You have 4 nights a year at accommodation options X, Y, X". So you aren't guaranteed any specific locations or days ahead of time.
What GP said is true if it's a single place. Those are typically bad because similar accommodations tend to be cheaper, the timeshare tends to depreciate, and the market isn't very liquid.
The ones that offer you nights per year are hard to redeem because they intend to sell to capacity. If it's a single unit, they want to sell all 52 weeks out of the year. You're typically competing with all the rest of the buyers to get a "good" week like Christmas or Thanksgiving. That can also tend towards being a bad deal if you're paying something like the price averaged over a year but you can only get in during the off-season when it's too cold/hot/rainy/etc.