What it means, is that in one of the xhtml files there is a reference to a resource. However, that reference is not known in the OPF file.
Sigil creates the OPF file upon saving the epub. It is based (among other things) on all the items specified. That's the list on the left side. It could be anything, an image, a font or something else.
Please keep in mind, that names and references are case-sensitive. That little tip could save you a lot of time and headache.
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