The IIIF extension is an extension to MediaWiki which offers a number of API services and associated tools for working with IIIF (International Image Interoperability Framework), such as integrating annotations, creating collections and using MediaWiki image files. Some of its features are based on Semantic MediaWiki.
Documentation about installation, configuration and usage is or will be found on the CODECS website : https://codecs.vanhamel.nl/Show:Lab/IIIF.
It does not include Mirador, which is part of a separate extension, but the extensions can be used together.