Folding causes multiple lines of text to be collapsed and displayed as a single line. It is useful for hiding portions of a document considered unimportant for the current task. Folding is purely a visual change to the document: the folded lines are still present, unchanged.
A fold is persistent. Once created, a fold can be opened and closed without needing to re-create it. When closed, folds can be moved over or yanked and put as if they were a single line, even though the underlying operation will operate on all of the text underneath the fold