The "crown" is the part of a tooth visible above the gumline. We liken it to a 2 story home.
Sometimes too much tissue covers up the 1st floor of the home.
An issue, like a cavity, below this tissue cannot be repaired since it is inaccessible. Trying to repair it will cause a chronic irritation of the gums.
To place a new crown or filling on a tooth, "Crown lengthening" is done to provide more tooth structure for a successful outcome with no gum irritation.