The meaning is the literal character '*' . fullfile() does not process the arguments: it just puts together a file name using the exact strings you pass in. You could pass in nonsense strings and it wouldn't care.
Now, the function that you pass the completed string to might care. The context suggests the string is probably being passed to the dir() function. dir() would pass the string to the operating system, which would interpret it in operating-system specific ways. Usually '*' is interpreted as a "wildcard" meaning "any number of any characters, but excluding the directory separator". In unix systems, the "null character" (decimal value 0) is also excluded.
