It is possible to protect bookmarks with a password using Link Lock, but a link that needs a password may appear suspicious to someone else seeing it. Hidden bookmarks solve this problem.
Hidden bookmarks are disguised to be identical to normal bookmarks, with one exception: clicking them in the right order will open a hidden link. To open the hidden link, click the disguised bookmark first, and then click the decrypt bookmark next. The same decrypt bookmark works for all disguised bookmarks.
Read more about how hidden bookmarks work here.
Here is how to create hidden bookmarks:
gmail.com
unless the current page is a disguised link. Use the "advanced" options to have it go somewhere else instead.