This is a metadata file that will make the nsxiv package show up in gnome software, kde discover and other appstream compatible software centers/stores when included as part of the package as done in the solus nsxiv package.
The metadata file alone is page.codeberg.nsxiv.nsxiv.metainfo.xml.
The Makefile provides {install,uninstall}-metainfo targets for installing and uninstalling the appstream xml file.