Usually when dentists cut teeth for crowns, they will pack a piece of string, known as dental retraction cord, gingival retraction cord, or just dental cord between your tooth and your gums. I’ve also used retraction cord when doing fillings on teeth that have a cavity that goes below the gum-line.
This cord helps move the gums away from the teeth and can also be treated with a solution that prevents the gums from bleeding. That way, the dentist can focus on preparing the tooth without the gums getting in the way. The dental cord also helps the dentist make sure that the whole tooth gets recorded when an impression is taken.
If you want to see what impression cord looks like in the mouth, scroll down to Figure 3 on this page for a picture (the dentist has prepared a tooth for a type of filling that will be made in a dental lab.) This article has another picture of dental retraction cord.