Have you got tons of email in your gmail account? Is it hard to find that one email that you have amongst all of those emails? Do you need to search for an unread message?
"is:unread" is the term you want to add to your search. So, let's say you have a label (folder like thingy) called ABC and you want to read all of the unread emails in there. You could put this into your search box on gmail:
That will do it.