If I visually select a series of lines -- say the open and close tags of a
large div in an HTML file I am reading through -- and then hit zf, it
will be folded into a single line. That line will list how many lines are
included in the fold as well as the content of the first line of the fold.
If I later come back to that fold and want to expand it again, I can hit
zd to delete (or undo) the fold.
To do this, you'll want to make sure your foldmethod is set to manual.
This can be done by running the following command:
:set foldmethod=manual
See the vim helpfiles (:h fold) for more details.