A wildcard is an asterisk character (*) that serves as a placeholder for text in searches. For example, searching "mail*" would bring up results with "mailbird" and "mailman" contained. The utility of this speaks for itself. You may want to search for emails with similar text, or (in my case) create a filter to send all emails from a domain directly to spam.
As more of an FYI, I will also mention regex, or regular expressions. Regex is a pattern matching specification that would allow for complex searches and filters, but may be much more complicated to implement. Wildcards should certainly come first, but it would be nice to see regex down the line.