Wildcard characters are special characters that you place in your browser identification string to indicate many possible values which may be present in that part of a string.
For example, the '*' wildcard character means "any character or characters". So that "Mozilla/4.*" means that after the "." character, any character or characters would be a match. "Mozilla/4.", "Mozilla/4.01", "Mozilla/4.0b2 (WinNT; I)", and "Mozilla/4." would all match that string.