When fans or the actors refer to 'guarantees' they are talking about the minimum they get paid.
Guarantees mean that they are contractually obligated to the show for that many episodes per week.
If they aren't used for all of those, then they are still paid. In the 80's, guarantees represented more episodes and there were a lot of actors who had some pretty big 'pay or play' contracts. That meant that they got paid whether they worked or not. There are people on staff who's job it is to make sure that all contractual obligations are adhered to. It is simply a cost of doing business.