Perhaps you wanted to get a list of contacts that have no first name. In the first name field, you would choose “doesn’t contain” and then the criteria would be an underscore so it looks like this example:
Alternatively, if you wanted to get a list of all contacts who you had a first name for, you would choose “contains” and then the criteria would be an underscore.
The underscore essentially means any character. So your searches are “doesn’t contain any character” or “contains any character”.
You can run this search on just about any field, although I’ve found it didn’t seem to work when searching for contacts that had a phone number!
Play around with it and see what you discover.