It means not available. Reasons seem to be various.
I downloaded Halliwell's Nursery Rhymes of England. All the text is there, but the illustrations got the red x treatment. The illustrations can be downloaded one by one, but not together with the text.
Some Bodleian broadsides show at low magnification, but the red x comes up when the magnifier is selected. Perhaps the enlarged image was poor, or they have not processed it yet. Dunno.

Some sites have treated images of music under active copyright in that way.

In some cases, the website manager has not yet entered the information.